Either the horse feels good and is just showing it, or perhaps a horse fly has attacked it and the horse is trying to get rid of the insect by bucking. and running around. Horse flies are nasty creatures. Out where I live, we call them the B-52 bombers, because they make a loud buzzing sound when they are nearby. They have a mouthpiece which is as sharp as a hypodermic needle, and when they bite, it's extremely painful to the horse. And these bugs will always land in places where the horse can't get them off, such as on the animal's back or their rump. That in turn is what causes the horse to run and buck. The horse isn't being mean- he or she is trying to escape the pain of being bitten. This is most likely what you observed this morning.