In psychology, the ‘foot-in-the-door phenomenon’ is the tendency for people who have first agreed to small things to later comply to much larger things.
This is how abusive relationships work. They do not start out violent and brutally abusive. They warm you up to it slowly. It is a psychological wearing down of your sense of self-worth and safety. Do not ever think “why did they start dating him?” or “why don’t they just leave?” because victims of abusive relationships have been psychologically conditioned to accept, or even think they agreed to and deserve the abuse.