Do you mean pike before the hands hit the horse, or after?
If it's before, it's probably coming from from not driving the heels back in the takeoff from the springboard. Drills for the heel drive action - heel drives laying down on a block. Not getting the feet in front of them on the springboard can also contribute - for that I'd make sure they're hurdling from far away enough. Use a visual cue to make sure, like a line or a mat on the ground. Can also do drills with getting the feet in front just with a springboard by itself, not the whole vault. They should feel like they're leaning back slightly when they hit the springboard.
If they pike after the hands hit, it can be because the vault as a whole is slow so they're trying to get over. For that - running drills, fast heel drive drills. Or because they're trying to see the landing - in which case have them look at their hands.
