Maybe she suspected her son of trying to steal her money. Also, people walked around with more cash than they do today. There were no credit cards then, and banks were not as safe as they are now. People used to pay for things in cash, borrowing was rare.
Interesting points. Still, that's a lot of money to be walking around with, and I'll bet there were muggers and thieves back then too.
I would bet she was BiPolar / Manic Depressive.