Is it a new habit or he has always been doing that ? Anyway this is dangerous for you and he needs to be reeducated. You can maybe try to desensitize him ? He might be ticklish but the reaction you describe is very violent. If it's a new thing it can be a health problem : a sign of ulcer, or udder infection or injury. Check for any physical issue that might be causing the behavior. Horses do not just change their normal behavior for no reason. Maybe calling a vet can be a good idea if it's not getting better after the desensitization. Let us know if it gets better :) Good luck !