So you just brought your new horse home. The person you bought the horse from didn’t seem to have any issues when riding him, but when you brought him home, he started to buck. What changed since you brought him home to your stables? Katherine Blocksdorf of About.com suggests some reasons that your horse may be bucking when you try to ride him:
Equipment
Before we get into the more complicated reasons, we are going to start with the simplest. If your saddle isn’t fitting correctly, it could be causing pain in the horses back. You should never just “throw one on.” Make sure that the saddle fits properly and your horse doesn’t mind it. Doing this alone, may be enough to alleviate your problem.
Outdoor time
Perhaps the previous owner kept the horse in the pasture and you’ve been keeping him primarily in the stable. This can cause a horse to get restless and try to get rid of its energy by bucking. If this seems to be the case, give your horse more outdoor time to help it exercise.
Riding style
Some horses just react differently to the person that is riding them. You may have a different style than his previous owner. Be confident in your riding and your horse should come around to accepting you as his rider.
Overall, the best thing to do is soak up all the information that you can about the horse from the previous owner. If the horse was primarily outside, but you want it to be in the stables more, be sure to keep things as familiar as possible at first. Let the horse get comfortable with his new surroundings and then slowly transition him to what you want the new situation to be. This will be best for him as well as for you and your riding.
Source: About.com