Honestly, I don't think horses can reason like humans, i.e. they don't sit in their stall thinking "okay, so I was lame today.. and she didn't make me work hard... so if I act lame tomorrow, she won't make me work again!" They just simply don't have that cognitive ability.
HOWEVER - pain can have a learned response. If something has hurt the horse over and over in the past, they can learn to anticipate that a certain action will bring pain, so he could be bucking out of the fear of pain.
A human example is: if you sprain your ankle, chances are you'll favor it for quite a while even after it heals, because you're afraid that putting 100% weight on it will hurt. This is a learned pain response.
Horses can do the same pain response.
I would bring him back to work slowly, ensuring that he realizes that being ridden won't hurt him anymore. If he continues to have the same reaction, I'd get the vet back out (or a chiro) and make sure that the problem is 100% taken care of.