the pony is probably proud with himself for clearing the jump and then does a buck of excitement!!

It may just be he is excited about jumping, but if you are doing it for like 20mins or more, you would think the excitement would wear off!!!  If he is a youngster than it might just be he is inexperienced and it is just soo much fun. You should get your tack checked to see if he has changed shape or whatever or if it is pinching him/her anywhere, but also get your vet out to check his back incase you are starting to make a small problem become a big one and then find out that becasue of it he can't jump or he has to be in very light work for the rest of his life!

hope this helps and good luck!

If we are confident it is just high spirits, how can we cure it?  It is a dangerous habit when there is a wobbly middle-aged person on board!