He could have had a nice frolic prior, and hurt himself. Something new has happened, and its causing him to buck. Horses just don't change their habits out of the blue for no reason.
If its not a health issue, could be a trainer issue. I'd not worry about confronting until a vet gives the a-ok that he's fine. If he's fine, then politely ask the trainer what she did. Again, improper warm up might have been it. And it could have just been that one time.
The trainer says he's never done it before, so I'm willing to bet its a health/cold issue.