Indeed, good observation. It could be any of the above or combinations of reasons. Tinyliny brought it to light as well. If a person doesn't know how and where and when to move their feet, surprising things could happen! It's often overlooked that the horse mirrors your intent and energy in his responses. It could be likely that this is carried-over from prior owners, environments and experiences. To break the pattern you may have to be firm and command a lot of attention (respect), but without being aggressive yourself thus setting it up to get the same thing back from him. Sometimes when you tag one on the butt with the popper on your lead rope, he kicks you.
Can you protect your life and your space when things get dangerous, but stay as calm as if you were just brushing him on a sunny day and everything was good between you? That's the very difference between getting one good as a result of your correction or confirming the behavior even more.
I appreciate you giving me so much credit, but I wasn't really taking that as far as you thought I was. only that the kind of "throw the bucket " at him reaction might create more of an explosion than a person would feel able to deal with in a stall, and could just end up in someone getting hurt.
I know I would have trouble disciplining a horse that threatened my life and wellbeing, without becoming angry and aggressive myself. it's a very natural reaction. horse causes me fear, I find myself inraged. Some folks run, and some folks fight back. i'd likely fightback, and it could work, or could make the horse more aggressive. I know it's not helpful to get enraged, but fear like that is hard to keep in its' place.
I am glad I don't have to deal with this sort of behavior.
A few times that I've brought grain to Mac, and he's tried to push in, I set the tone at the very beginning, just when he starts pinning his ears and starts to thinking about pushing in on me. I make him move off. he isn't "happy" about it, but he moves of. but he is pretty textbook as horses go.