What could be the cause of my father's mood swings?

OK.. he grabbed a knife?? WTH? Next time you call the cops or EMS or you or mother may be the next to need it! Something besides depression is going on here. Get him to a Dr or evaluated at the hospital

He needs help. If you can't get him to help, you and mom need to move. You are enabling abusuve behavior by remaining. I understand your not wanting to leave your mom in danger but time won't cure this will it?

Your mother is probably suffering from battered spouse syndrome, and is afraid not only to stand up to him but also afraid to leave. That's certainly not her fault but she needs protection.

Has anyone reported him to the police? That might be the way to get him out of the house so you can remove your mother, while also asking the police to help get a PPO to restrain him from harassing both of you.

If he's arrested, the court can order a psych exam to find out what's happening to your father.

It might also be that your father has some awareness of his situation and is afraid/angry that he's losing control as he ages, if this is the case. I'm guessing he's in his 50's or 60's? Has he had any major illnesses, physical or mental?

I'm sorry to read of this situation; it must be so sad and confusing when a father to turns into an abuser, but do think of you and your mother first.

@GardenArtist  he's never physically hurt my mum - and it's the first time he's ever hit me. He just has a way of making everything about him, he's always right, he always has to be in control. it's impossible to criticize him, or even joke around with him. He loves teasing and making fun of you when he's in a good mood, bu you can never do that to him. If you do anything to upset him he will punish you with grumpiness and the silence treatment that can go on for days and days. If you dare say anything he get's very angry and swears and makes threats of violence. We are always on egg shells. We have to constantly watch what we say and do. It breaks my heart as I love my father so much. He wasn't like this before. He was such a great fun dad. I just am so sad to see what he's become. I just want him to goo back to being happy. Ans I want my mum to let go of him instead of making excuses for him. He refuses to go to a doctor, saying that it's us that needs therapy. I just can't bring myself to give up on him. I just want there to be away of him seeing what he's doing to our family. If I called the police what could they actually do? mum wouldn't want me to but if I did would they even take me seriously? He doesn't physically do anything.

He said he would use a knife on you.. they take that seriously! And next time he might

Swings in blood sugar can make some people nasty (although threatening with a knife is extreme). Try getting him to see a doctor.

Is there any way you can make a recording or video of your father when he is behaving this way? It would be great if this could be presented to him when he is in a good mood to show him how he changes (although this might "set him off"), but more likely it would be useful to show a doctor or other authorities how he behaves.

29yrs old and living at home? Put on your big girl pants and get out of that situation and out on your own.

Testdepth, that wasn't a helpful comment. She said she doesn't want to leave her mother, her mother needs her support at this time. Her leaving would not solve his problem or her mom's situation.

Okay. I recently read a book by a daughter who went through exactly what you were going through, except much worse. If this is the same thing, her father was going through dementia combined with a life of being handled with kid gloves by his wife and family, lauded by their circle of friends. Many people do this. I have a husband I do this with. The problem is he always had an anger problem but now it is not only combined with fear from loss of his control, but also dementia setting in. He can probably become worse down the road so you need to deal with this now. He needs to see a doctor, the right doctor. If the first one doesn't work out, see another. You must be wiley and manipulative to get the care he needs. I am trying to find the book, will get back with you when I do. If you can get him on film, that would be great - people tend to not believe you until they see it. Take the film to the doctor.

Keep a journal.

SU 19102017, would you mind doing yourself a favor and reading what you wrote to realize the severity of what you're describing? Sometimes a situation can snowball to dangerous limits but because we're living it we don't recognize how serious and dangerous it has gotten! That's why I'm asking you to read what you wrote, as a third person, to realize how bad is bad based on what you're sharing.
Then realize you need to take action immediately.

I'd say that your father suffers from a classic case of Narcissism (read up about narcissistic and borderline personality please, you'll understand where I'm coming from), but the thing is that you say he wasn't like this before, which tells me there's something else going on. I think he definitely is suffering from either a mental illness (could be dementia combined with depression and anger), or something else is really out of balance physically; someone mentioned sugar ups and downs, also could be his thyroid -a thyroid out of balance does crazy things to a person's body and mind-, there are so many other potential reasons! But neither you or I are doctors.

So to me, the solution goes in order of relevance. Number one is safety, so your mom and you need to protect yourselves as much as you can, and you also need to get your mom onboard with helping your dad WITHOUT risking your life and sanity. That's also protecting him, because if he does something bad he'll end up in jail.

Second is helping your dad. He definitely needs to be seen by doctors asap! looking for mental and physical illnesses. Once you explain the degree of severity to any doctor (please mention the knife, hitting, etc) they might help you find options we aren't thinking about.

Are there any relatives or friends that might help with the situation? Maybe talking to him to convince him about medical help, maybe not saying for what really, just a check up or something like that.

And can you and your mom go for a trip for a while? just to be removed from the situation, to protect yourselves and to come up with a plan. Your mom needs time away from him to think clearly. Also, while the situation is not solved and when you are interacting with him, don't contradict him, don't joke around, don't even participate in a conversation as you'd usually do, even in his good days! Switch your mindset about him from "your dad who is acting crazy" to a person who's really ill and dangerous.
Just keep conversations on the surface, and if he starts getting mad, either remain calmly in silence or say I see your point dad, without sarcasm or anything. Realize you're not dealing with your 'normal dad'.

And lastly, start turning your resentment towards him into compassion (always aware of the danger though); remember he is not well. He doesn't mean to hurt you, he simply is not well. So in your heart and mind the image of your dad should,remain untouched!

God bless and good luck!!

PS. For who said you're 29 and should not be living at home, I think you are there for a reason. You can now help your mom and also your dad. Everything in life has a purpose and a reason!

If possible film it with your cell phone a few times then as you are a resident of the home call cops to have him removed - he is possibly a narissistic sociopath [NS} with the beingings of dementia -

When in doubt document, document then again especially as he has everyone hoodwinked into believing he is great - educate yourself as much as your can - there are sites about NS as well check out Deeepa's Gems -

With some education & knowledge behind you then when you need to act you will be speaking words with clout [like 'dementia', 'sociopath' etc] not just saying 'Dad's acting weird' - you then immediately become someone more in control not just a complainer

Good luck & keep us informed

su19102017: Here in the United States of America Domestic Violence is an offense whereby the police are called and the perpetrator goes through the justice system. No one should tolerate Domestic Violence! Get a restraining order to protect yourself and your mother. They can get your father help through the  justice system.

Please try to get him to a doc. Some people have said I need to go to the Dr. Please come w me & guess what it's not for me & before you go make sure your videos & or daily diary are seen by the doc. Anti depression med turned my alz hubby from lion to lamb. Amazingly our children thought he shld have been on them way earlier but he was in control & nothing wrong w him, so he thought. Besides he was the educated one & what would I do or support myself/children so we stick w them & God helps us thru. 10 yrs of alz & he is so much more loving & thankful. Wish he'd been that way when we were younger. Call Alz 24/7 line.

@su19102017 Please act quickly, for your mother's sake. You are young enough to be resilient and recover, but she will have old coping habits to change, and may be feeling as protective of you as you are of her, so to get both of you on the same page, you need to take your mum away on a short break. Then sit down with her, use the argument that your dad/her husband needs medical help and tranquilliser meds, and don't go back to the house until you get a plan of action for getting the right doctor to visit the house. (Getting him to a doctor is probably an impossible option.)
Narcissism is extremely difficult to handle (look at Donald Trump) but both you and your mum must stand together - don't let him divide you and belittle each of you.
I am the child of very loving parents who sacrificed a lot for their children. My father was physically disabled but mentally tough and serene. I cannot imagine what you must be going through. Courage!
And ignore the voices of people telling you to move out. It must be both you and your mum together to somewhere safe.

Su, we have many similarities between our fathers - fun and charitable on the outside, a raging bull to me and my mother. My guess is your father has always had mood swings and you were brought up (as I was) to believe that was normal and acceptable behavior. 

At 94, my father was (as he refused to complete the test, but already scored high enough) “unofficially diagnosed” with borderline personality disorder, highly narcisstic and little to no impulse control. Fortunately, a kind nurse called to share this info and urge me to protect myself. (He is the type of person who believes discipline/whippings solves everything. This has not changed with age.) Because of his age, they would not prescribe meds.

Barring getting him to a doctor for meds,
1: find a great therapist for setting boundaries (I, too, have walked on eggshells all my life; now I’m stomping them,)
2: rely on outside help as much as possible, and
3: get a small handheld container of mace or pepperspray for you and your mother and don’t be afraid to use it in case you can’t call 911 fast enough to prevent him from injuring you or your mom.

Courage and strength to you.

I would get out of that house. Your mom has already adapted to your dad's character by being an enabler throughout her life. She'll not leave him. Your staying doesn't help you or your mom. It helps only feed your insecurity and your dad's narcissism. Get out before you become your mom!

Su191: Dad’s rages are not new behavior (“realized...how much abuse Mom has taken over the years”).

I suspect you are an only child? It is typical for Mom-Dad-Only triads to somewhat operate “in their own world” — and to parentify the child to some extent. Even in higher-functioning families.

In your case (I came from something similar), you grew up knowing you weren’t like Beaver Cleaver’s family, but it was YOUR normal....and you automatically fulfilled your role as the balm and the deflector.

Then you got out. Got a taste of how to function without being anyone’s buffer. Voila — you got to just “be.” That’s hard-won, when you spend your childhood and teen years as a pawn.

Now you are back in the fold, but with the perspective of a mature adult. Your parents are firmly entrenched in old patterns.

Mom and Dad are fine with making their trainwreck your problem. But you are NOT fine with it. You learned too much, you’ve grown too much.

Paradoxically, your personal growth is also a loss. You lost your Daddy’s Girl filter. And rightly so. Nonetheless, it’s a huge shift in perpective. Redefining old memories can be painful and uncomfortable.

Dad’s escalation to physical violence is a real concern. Maybe he has cognitive/neuro issues.....maybe not.....maybe your Mom will break down and confess that he’s been beating on her for XX years.

Previous posters have given you stellar advice on how to respond to Dad’s rages; involving law enforcement as needed; your personal protection. Follow their lead.

Good luck. Stay strong and vigilant. I’m not sure what landed you back home at 29, but please do not make it your life’s work to referee your parents’ mess. 

Be safe. Detach with love. Move out as soon as feasible. (((hugs)))

I agree with the posters: you need to get the police involved if he strikes you or your mother again - the mere fact he went for a knife is scary enough. He could be in the stages of Alzheimer's/ Dementia, it could be a lot of things, but it impetrative that you find someone that can diagnose it before either you are harmed or your mother. Believe they can hold up a façade to the outside world; they don't live inside the home; you and your mother do. You need to protect yourselves and him; but walking on eggshells or justifying the seriousness of the situation will not help anyone at this point. No need to become a statistic because they only can manage for so long before everyone sees; so the sooner that help comes; the sooner you all will feel better for it. He may have an undiagnosed mental illness but I'd call around now and see what you can do to protect yourself and your mother and make sure you're safe before deciding anything to your father, even if its to see a doctor (which is not likely he'll go to.) The priority of care needs to go to you and your mother; had you not mentioned he stuck you and grabbed a knife I'd have suggested other means but he doesn't need another opportunity to do that twice and you don't need to give him time for figure out when to make his next move.

Is he taking opiods for pain or tramadol? Sudden withdrawal or inconsistent use from the aforementioned psychiatric meds or benzos or both may contribute to his behavior.

As I've posted here, DO NOT TOLERATE DOMESTIC VIOLENCE! Good grief...it may be the death of you!