My mother is increasingly crying all of the time. When I ask why, she says I don’t know. What can I do?

Pseudobulbar affect is not uncommon with people living with dementia. PBA can manifest itself in uncontrollable crying or laughing that doesn't fit the situation. Nor does it mean that the person is even sad. There are meds that can address this condition so see her doctor.

Have you alerted her doctor to this change in mental status?

That is really where you need to start. A UTI should be ruled out, medication interactions and the like. There is also a condition called pseudobulbar affect that can cause this.

https://www.mayoclinic.org/diseases-conditions/pseudobulbar-affect/symptoms-causes/syc-20353737