Although I was bullied in school, for me it's more of an abusive step-dad fetish I guess.
I barely knew my dad and Mom had bad taste in men. She had a lot of boyfriends and although most of them were somewhat decent guys there were a few that were truly awful. I've seen my mom get knocked around more than once and have been knocked around myself as well. It's horrible being at the mercy of some strange man you don't even know who comes into your house and does whatever he wants to your family. Feeling scared and helpless, unable to protect the people you love let alone yourself.
Obviously I hated it then, but as an adult I can't help but sometimes get turned on by what I was forced to endure growing up. It's the negative emotions I focus on; the helplessness, the humiliation, feeling so small and weak and preyed upon. I guess that I crave that abuse that I no longer get as a grown man. Thinking back on my mom getting smacked around or my younger sister getting touched or what was done to me just turns me on in a way I can't explain.
As I mentioned I was bullied in school, so I guess imagining bullies in the place of my mom's abusive boyfriends works and creates just enough distance for me to enjoy it. Who knows why this turns me on.