I have two physical reactions that I didn't understand for years. I knew the trigger, but not why.
The first one I discovered was when I was getting my hair cut. I found that when the razor got to a certain point on the back of my neck, that it caused such a sensation that made my leg kick. It's hard to stop this, and I have to focus to prevent it.
My stomach has massive touch aversion. Not touch aversion from myself, touch aversion from other people. Even my little dog can trigger it. It twitches and sucks itself in, I can't control it really. I've wanted to freak out and just yell at people trying to tickle me, but I couldn't bring myself to it - I'd have to explain why, and I just knew they wouldn't understand, and I didn't want anyone I knew to know anyways. Instead I would just defend myself, and tell them I didn't like to be touched (which is true anyways).