2) Even if you tried ONE other saddle, it could still be a saddle problem!
If your saddle doesn't fit, it can have caused muscle bruising down deep in the tissue, so even a "correctly" fitting saddle may evoke the same response because of the bruising. My friend had to give her filly 2 weeks off due to a deep tissue bruise caused by a too-tight saddle.
Often the canter will be "the" gait that a horse will give you the thumbs up or down on a saddle - because it's the only odd-beated gait, and it rocks everything forwards and back on the horse's back. I had a friend who must have tried dozens of saddles to find "the one" - and her mare would let her know in the canter. She'd be fine at the walk and trot then blow up in the canter if her withers were being bothered.
A sudden-onset bucking issue always makes me think "pain" - and that is most often the case.
Do you have video of her moving? Another issue can stem from the stifles. Does she cross-fire?