And to the poster that said it can't be pain because he would have been bucking at home too if it was pain...you bet it could all of a sudden manifest into a buck at a show and not at home first! OP already stated it was a cool fall morning and got on to school the pony, even though that is not the norm, in case he was a bit fresh due to the weather. I find the show ring heightens things as compared to schooling at home.
If the saddle fit had been getting progressively worse and he's just been putting up with it at home up until now, a show, being a bit fresh on a cool fall day and getting more saddle time all day as compared to, say, a one hour lesson at home could certainly make him state his case loud and clear. Sure, some horses and ponies would try and warn everybody by head shaking, tail swishing, etc. first, before progressing to the bucking stage...but some do not! And, some do try and communicate with subtle signs first only to be completely ignored by everyone. It's possible he was tail swishing or showing other signs and it went unnoticed.
And please do not get a "pro rider" out to check saddle fit. I know many top pro riders who think they know saddle fit...but really don't know everything about the technical side of saddle fitting. You need to get a well known, well respected saddle fitter out who has credentials and lots of experience.