You have gotten really good advice -- so look at saddle fit, possibility of ulcers and possibility of needing a good chiropractor.
Then, just consider how many horses start to buck: They are ridden 'fresh', as in feeling really good on a cool day, maybe not getting a good enough 'warm-up, maybe having a chance to buck and play on a longe line. There are about 20 maybes that a horse can learn a bad behavior from and then the horse lets it become a bad habit. He finds out that if he 'threatens' to buck (or really bucks or crow-hops), his rider will pull him up. He figures out it is a good way to get out of work. He can now make his rider pull him up. Then, of course, when the rider tried to ask for a canter or ?? again, he threatens to buck again --- and the whole thing starts to spiral out of control. Soon, the 'once nice' really broke horse has a bad habit that can get to the point of being dangerous and having the horse get totally out of control.
I have seen horses start out with this behavior and end up in PRCA bucking strings. They learned it; they liked it; they got really good at it. Most horses just get a little more 'cold backed' and people learn how to compensate for them as they do not know how to completely make the behavior go away.
They start out with a well broke horse that they can just catch, saddle up, get on, and go ride safely and pleasantly.
They end up with a horse they have to longe, round-pen or exercise for 30 minutes before they can mount and then may still get their butt bucked off. They end up with a horse that is waaaay over their heads or at least, their kids' heads.
Am I getting anywhere close?
If this is what your case sounds like, let us know and I will tell you what I do to get the 'cold back' out of a usually nice horse.