We have a horse at the TB Rerun I work at that does the same thing. We figured it was because he was getting excited/anxious to ride because he figured bridle+person=racing.
Before you put a bridle on, work him in the roundpen at just the walk and trot. He needs to know that it's easier to go slow and that's what you want, is for him to go slow. It might take awhile but once he learns your commands then move up to the bridle. I would use a regular snaffle, training ones will encourage him to play with the bit more. And some horses will never outgrow the habit, it's not the worst. When you first put it on leave it on for 5-10 mins and take it out and reward him by saying "good boy". Then do it again the next day.
Just take it slow :)
Last edited by White Foot; 05-26-2010 at 08:34 AM.