Sounds like my 100+ yo mother! Although she's very sharp in many ways, I see changes in her behavior that I find unexplainable.
Same here with my 85 y/o Dad. I think it sometimes has to do with the ongoing and very visible loss of control over their lives and their physical environments. My Dad may say, I can’t walk as I used to, and I can’t drive a car either, but BY GUM I’ll be DAMNED if I have my children tell ME to move out of my own house!!! Never mind that the family house is literally falling apart. We stopped fighting it, my sister’s and me. We will enjoy my Dad on his terms while he is still here. I don’t wish to be on anybody’s Sh#t List. He does hold on to his grudges! Sometimes I catch myself doing the same thing.