Is this a TB rescue by any chance? I ask only because they are notorious for this (as well as not standing at the mounting block). It is not because of the breed -- it is because at the track, they do pretty much everything fast. They're often mounted while practically moving and expected to go forward immediately. Grooming is perfunctory. Standing still is quite literally unheard of, being tied down is REALLY unheard of.
Obviously many horses of all breeds have this vice, I just know from my own TB that they can be shockingly 'untrained' for an animal that, say, could care less about a giant fan or motorbikes or flying hoses, but will melt down at the prospect of a shovel in an arena.
Overall, my point is -- it may help if you can kind of feel out the source of the distress (Is the horse just fussing? Is it genuinely befuddled and upset by being restrained? Is it sheer pig ugly stubborn?) you'll have a better shot at fixing it.