There are numerous causes for bucking. The first one to rule out is that the horse is being irritated by his tack. This could be poor saddle fit, a bunched pad, or an uncomfortable or pinching girth. Try different types of tack and take a close look at how your saddle fits your horse.
The next cause for bucking is that your horse is scared; perhaps he was not started correctly if he is a youngster, or he has had bad experiences while being ridden such as riders falling, that has given him a new fear of being ridden. If this is the case, go back to desensitizing your horse on the ground by slapping the saddle, moving it around, etc. Then practice mounting slowly, and laying over the horse, making sure that he stands still as you do this. Get on and off a few times, and ride forward being aware of your horse tensing under you.
Bucking can also be a bad habit that a horse has learned gets him out of work. Either he has learned to throw his rider, or learns that if he starts crowhopping and acting silly, the rider will get off or at least stop asking him to work. To determine if this is your horse, ask yourself if there is a pattern to his bad behavior? For example, does he buck when you pass a certain area of the arena, or does he buck when you ask for the canter? If this is the case, then the horse needs to be pushed past his bucking by an experienced rider who is capable of keeping the horses head lifted and pushing him through a few jumps or bucks.
Excess exuberance can also cause horses to buck. If it is a windy day, your horse has been kept inside, or the weather just got cooler, he may simply be feeling good and needs to release some energy. If you feel that this is the cause of your horse’s bucking, then work him from the ground or turn him out before you ride. When you work him from the ground or on the lunge line, allow him to move forward, but do not allow him to buck in hand or on the lunge line. He needs to learn that bucking is not acceptable. You can either push him forward or make him slow down if he starts to buck on the lunge line.
What to do if you are riding a horse that bucks? First, ride defensively, sit on your butt and do not lean forward. Push your horse forward, if your horse is moving forward briskly, he has to first stop or at least slow down to buck. If he feels tight and behind your leg, it is easier for him to act up.
Bucking can be a very dangerous and intimidating behavior, if you are still unsure of why your horse is doing this, or nervous about your ability to correct it, then consider consulting a professional.