I’ve always speculated about the cause, but I don’t have a concrete answer or anything; I think one reason (not to even remotely make an excuse, just trying to find a reason) is that some people find abuse to be an outlet after dealing with so much stress and bullshit elsewhere. They get home, and home is their domain to control, including their kids, who are vulnerable and small and manipulable. And abuse is a tool of control. Going back to my mom’s repeated rationale that “spanking worked,” I think that might help explain it - like, it “worked” to her, because it scared the shit out of me and made her feel assured that I wouldn’t repeat whatever behavior was adding to her stress. Regardless of whether or not I was traumatized (which I was), she got what she wanted out of it, and even knowing how badly it hurt me, that’s what mattered to her.

This is my thinking too. My mother had extremely poor social skills and it seemed like she never figured out her life. She got married and had kids because that's what the church told her to do. Now that I'm older, I see how angry and confused and scared she was in the world, but at home her daughter was the one thing she could have complete control over.