Nothing.
Stay out of it.
It is between your parents.
My mom keeps getting mad at my dad because he does the same mistakes repeatedly and she gets violent. I get scared and depressed whenever she does that. What should I do?
It's affecting you, so you are in it whether you like it or not.
Tell your father how much it bothers you when your mother acts like that toward him.
If that doesn't work, try talking to your mother when she's calm.
Your Dad might be a stooge, but Mom has serious behavioral problems. There is no good excuse for initiating domestic violence.
That you live in fear of this is a problem that needs addressing. I suggest that staying silent will only enable the problem to continue, or worsen. You deserve a peaceful, civil home environment.