For me it was a bit of a let down that there was no real reason for the rape of Trish or any of the others.
I had a scenario in my mind that I was really, really hoping for. That Cath found out who Jim was cheating on her with and she raped all those women with a strap-on or something as a sort of revenge act.
But no, it was just a horny dude, who wanted to feel power every once in a while.