With the warmer weather, he could just be "feeling his oats," so to speak. Also, horses have a lot better sense of smell than we do. He could have been picking up the scent of a predator somewhere in the area (even one that had passed by hours before). I know my friend and the trainer were out on an endurance training ride last fall and their horses (both arab geldings) all of a sudden started acting nervous, dancy and almost panicky. The trainer happened to glance down at the ground as they went past this sandy/dusty area and spotted BIG bear tracks that were only a few hours old. The horses were reacting to the bear scent.