@hugo_hayes nailed it when he said to check your own two feet first. I've had that same thing happen more times than I can count, and it's almost always something simple I was doing wrong. A few weeks ago I had a mare that was throwing her head and dancing around until I realized I was bracing my shoulder into her hip too hard without meaning to. Soon as I eased up, she sighed and stood solid. We'd all do well to remember that horses communicate through their bodies, not their mouths, so if they're fighting, maybe we're just not listening to what they're saying.