On a lighter note, I watched a video on you tube of an amorous stallion and a round bale - quite hilarious. Sorry, I can't remember the title but check it out if you have some time.
On a serious note, I'd say he was having some sort of hormone surge and you and your hay bale were either the cause of it or you just happened to be in the wrong place at the wrong time. Needless to say, please exercise caution until you re-establish that you're the one calling the shots and not him - carry a weapon (eg whip, stick) with you when you're around him and make sure he keeps out of your personal space.
I'm curious - do you know when he was gelded? Does he have a stallion appearance about him? My initial thought when I read your post was that he was either gelded late, having bred mares before, or the gelding process left some tissue behind.