Horses are the only common domestic animal whose primary defense is flight. When threatened they assume a flight position with head and neck elevated, back rounded, hind quarters gather for a sprint, unless they choose to fight. The horse signals submission by assuming a position of its greatest vulnerability, when it is drinking or grazing. The horse, therefore, acquiesces by lowering its head, working its mouth, chewing and licking its lips, Experienced trainers recognize these signs and consider them to be mile posts in training. Just as chickens peck and dogs snarl and snap, and cattle hook, horses exert their leadership by controlling the movement of their subordinates.
Movement is life to the equine.
It is survival behavior. Horses are flight creatures. Watch a group of horses. See how the leader threatens those below it and cause them to move out of the way. Every time that happens they are acknowledging her leadership.