Why does Corinthians believe her father hits her mother?

Corinthians believes her mother purposely provokes her father to violence.

But Corinthians began to see a plan. To see how her mother had learned to bring her husband to a point, not of power (a nine-year-old girl could slap Ruth and get away with it), but of helplessness. She would begin by describing some incident in which she was a sort of honest buffoon. It began as a piece of pleasant dinner conversation, harmless on the surface because no one at the table was required to share her embarrassment; but all were able to admire her honesty and to laugh at her ignorance.