I often hear about horses with a wicked buck that routinely dump excellent riders. A playful buck after a thrilling jump is one thing, but if your horse is consistently trying to unseat the rider, chances are that horse is in pain.
Bucking is a defensive move, a way to protect from predators. If a mountain lion jumps on the horses’ back, bucking is a good way to get rid of it! If something hurts in the hind end, bucking seems like a logical way (to the horse) to get rid of the pain.
Give your horse a back massage. Tight muscles will hurt with the added weight of a rider. The long back muscles, lumbar, top of the haunches, and areas around the croup all should be relaxed, elastic, and pain free.
Saddle fit should be one of the first checks when investigating the cause of bucking.
Make sure the bit fits and is comfortable for your horse.
Be careful about feeding very “hot” feed until the problem is resolved.
Make sure your horse is getting enough time outside the confinement of a stall.