Does he get grain?
It sounds funny, but I've known horses who are completely intolerant of any grain whatsoever. It makes them so energetic that they just seem hyped up and crazy. We started giving grain to one of our geldings as a winter supplement. He only received a butter container full morning and night (I'd say somewhere close to 1-1.5 lbs a day). He's very well-trained, but the next time I took him out, he was just out of sorts and spooky as heck. It was very unusual, and very different from his usual calm, collected manner. When we took away ALL of the grain, he went back to his old self.
Feeding time can also be an issue before you ride. Sometimes feeding grain before riding can get them all jazzed up (depending upon the type of feed and their reaction to it). And sometimes it's just the type of feed itself. I was told that my mare ran through fences, and acted crazy when on SafeChoice. When they replaced the SafeChoice feed with Strategy, she calmed down considerably.