Horses don't only paw over food either. Mine did it because he was impatient with the situation. He'd paw when tied if he's not crazy about he location/situation at first, He'd paw in the trailer too but only did it until we began driving, then stopped doing it.
Twister would quit if you told him no and re-assured him that he was fine and meant to stay put.
It's a learned behavior to get attention from humans.