Why is my mom like this?

This sounds like a clear cut case of verbal abuse which if your under 18 is child abuse but in any right it needs to stop. If you confide in your father talk to him about what's going on and see if he can help. I would recomend family therapy. Usualy things like this are learned ie. Your mother was abused when she was a child and she never got help and she has become the abuser. In any case its not good for any one involved and help sould be sought as soon as possible.