Back to basics and after the jump, seat properly and pull up only one rein
also try a ground pole before and after the fence it will make her work harder and put her bain to work and make her thing about that and not bucking
if it’s not physical pain to the horses legs then try to keep them collected after the jump or halt them a stride or two after the jump. The same thing happened to my green horse he did that all the time after a fence but doing those two things really helped and now he doesn’t even try to do it
Yes you need to see if he doesn't have any physical problems with your vet first.
If there is nothing, it can be several things. If he's young and "green" on the jump, he is probably having fun or trying to release the pressure because it is difficult for young horses whose back is not fully muscled, he will calm down with the age.
If he's used to it, it can be a bad habit. Try to stop him right after the jump (you can even put a ground pole after the jump so that he has to stay focus even after the jump). You can also do a lot od mechanization lignes, he won't be able to "run away" and go crazy after the jump.
It can also be a problem due to your position, if you don't follow enough the jump with your hands, he will make you understand by doing it so don't hesitate to move your hands forward during the jump.
Finally, go quickly at your place after the jump because if you stay on his shoulders, it will encourage him to run and do stupid things like trying to put you off.
I hope i could help you :)
Hi Patrucia, did you check that your horse is not in pain? You could call your vet and your osteopath to make sure that everything is ok (check teeth, back, joints,...). Once that's done, you can start to think about ways to fix her behavior.
Hope this helps 🙂