A saddle is composed of some parts that flex and bend, and some parts that are fixed. The flexible parts may be adjusted, but the parts which are fixed must fit both horse and rider well. A saddle which does not fit a rider can cause problems with the rider's balance and stability. A saddle which does not fit a horse well usually causes pain. Bad saddle fit can contribute to "problem horse" behaviors such as resistence or acting out, bucking, rearing, "girthy"-ness, putting ears back or biting when saddled, not wanting to be caught or ridden, refusing to stand for mounting, and many more. A saddle which is narrow at the points may become excessively painful during posting.
After saddle fit has been adjusted, a horse may continue his previous behaviors for a while. He still expects the pain; he associates it with the saddle or with being ridden. You will have to convince him that the saddle will not hurt him any longer. Give him some time to get accustomed to the new fit of the saddle.
A horse who was often ridden in a badly fitting saddle may also have developed muscle problems. The saddle may have been digging into a muscle, or could have caused him to stay tense while saddled. If you have had a saddle fit problem, you may wish to consider an evaluation from an equine therapist.