Check for stifle issues! Stifle issues can commonly cause bucking. Horse Problems Australia talks about stifle problems in horses and their reprocussions
I have a mare who cannot canter due to some kind of stifle problem. I've had 3 vets look at her, 4 including the vet who did her pre-purchase exam. None of the vets found anything wrong with her. X rays were normal, injections did not help. What does help? If I ride her 3-4 hours a day for a week she will go (not sound), but sounder. I definitely believe it is a stifle issue at this point. If I ride her hard for a week, she can canter without bucking by the end of the week and jump small logs without a problem.
It is purely physical for my horse. She cannot bend to the right either. She gets really cranky if you ask her to do arena work and bend in a circle. At this point, she is strictly a walk-trot trail horse. She can trot all day without discomfort and is happy. Here is a video from a few years back:
Notice the cross cantering? and lead changes in the hind end? She is like this in both directions. If I ignored the issue, she could escalate to full out bucking. Notice how when I fix her lead, after a few strides she switches back to the cross canter.
There is another youtube video of a horse that looks similar to my mare. That horse was eventually diagnosed with kissing spines.
Start with a good lameness exam and check for back pain. A friend has a horse that "bucks someone off every 3 months but is good the rest of the time." Her horse also had stifle issues, where his leg locked up for the farrier.
So start with a good vet check and chiropractor visit! It could still be behavioral, but I would assume it is physical unless proven otherwise.