I spent a long time trying to figure out why. Read lots of articles, books, websites etc. All very informative.
But in the end, the most helpful work I ever did was figuring out my own "why". Why was in a relationship with someone who would treat me like that? Why did I not believe I deserved better? Why did I put up with that stuff from someone who professed to love me? Why did I not want to be on my own? Why did I make it all OK?
Because, really, it doesn't much matter to me why someone treats me badly. That's for them to figure out if and when they decide that their behavior is causing problems in their life. It's my job to look after myself.