Does he do it when you are practicing at home? Or only at shows? Maybe he's learned that he can do it at a show because you won't correct him. If he does it all the time, you just need to make sure you correct him every time he does it so that he doesn't have inconsistencies. If he only does it at shows, it's going to take correcting him as much as it takes in a class at a show once or twice. It will probably take you out of the ribbons, but it will be better in the long run.
To rein a horse is not only to guide him,
but also to control his every movement.