If the horse did not buck before you owned it you need to ask what has changed since the horse has been ridden by you. Ask what has changed about its environment, health, feed, the equipment you are using and how your riding may differ from the pervious owner's.
A change in the amount of time your horse spends in its pasture and stall can cause behavioral problems. Has the amount of time your horse spends outdoors changed? If it is accustomed to being outside most of the time, and it is now being stabled more than it is used to it may be expending pent-up energy while being ridden. Increasing turn out time will give your horse the opportunity to exercise itself.
Most of us like to feed our horses ‘extras',’ and some horses get more concentrates than they really need. Your horse may be ‘feeling his oats’ with more fuel than necessary. Most horses, ridden once or twice a week will only need good pasture or high-quality hay to stay healthy.
Have your horse’s teeth checked and consider having it adjusted by a chiropractor. Some horses misbehave under saddle because of body pain.
Poor saddle fit can cause your horse to misbehave and is often overlooked. If your saddle is pinching or concentrating pressure in one area, your horse’s back can be come very sensitive and cause it to buck. Be as careful about fitting your horse’s saddle as you would buying yourself new shoes. Saddles aren’t ‘one size fits all’.
Your horse could be objecting to having the girth over tightened. You should be able to slide your hand between the cinch or girth and the horse’s body. Over tightening can cause pinching and chaffing both in the saddle and cinch areas.
Bits occasionally can cause problems, especially if the horse has a dental problem. Be sure your horse has a bit that fits, is comfortable for him to hold, and is the mildest possible to control him.
It may surprise many people to know that horses don’t actually like being ridden or driven. Much of horsemanship is convincing your horse to do things willingly that it sees no earthly reason to do. Some horses figure out very quickly just how skilled and determined the rider upon its back is. And just like people, they can become very adept at avoiding work. If your cues are muddled, you aren’t able to read the horse’s thoughts and don’t know how to curb unwanted behavior many horses will pick this up.
When your horse bucks, it probably gives some signals beforehand. It is up to you to learn how to recognize these signals and take countermeasures before the bucking actually begins. This is where a good instructor can help you hone your skills. Many people spend many hours and dollars on finding quick fixes—buying more severe bits, tie downs, etc…But the answer to most behavioral problems under saddle is to increase the skill and knowledge of the rider.