My great grandma gets grumpy. Allot here is an example: My little brother was trying to record a show on are TV and she locked her self in my room for two hours.
Older people can get grumpy. I think much of it is because they feel vulnerable, which is kind of scary. They also feel like no one listens to them or likes them much. This makes them grumpier, which makes no one listen to them or like them. Whatever you do, don't ask her why she is so grumpy. That will just make her mad. And a grumpy great-gran is a lot better than a mad one. :-D
Frequent grumpiness at irrational things could be a sign of health problems.