Why is Mrs Dubose so mean in To Kill a Mockingbird?

As the children passed her house she would berate them for minor infractions in dress and decorum (or nothing) and, especially, put Atticus down for his defense of Tom Robinson.

I forget why they had to go that way, or why they could not get by without being noticed by Mrs. Dubose.
She speaks her mind and can be very rude. She makes a comment to Jem about Atticus defending a black man.
because she makes the kids read to the mockingbird.