My Paint mare can give me an infinite variety of canter styles. Sometimes it’s lovely balanced collected. Other times she falls on the forehand and does what I call “pronging” behind, and it can feel like a cross between a jackhammer and a buck. For her, the solution is to ride forward, and once we have impulsion, that pronging canter is gone for the rest of the schooling session.
I have seen what it looks like from the ground with other people riding, and have asked: does it feel like she’s trying to buck right now? The answer was “yes.” But from the ground it’s clear she isn’t bucking at all, and it doesn’t even look that bad, only that her hind legs aren’t really under her.
Lots of horses can get on the forehand in the canter, and it’s never a great feeling, but IME a longer strided horse gives you a different sort of forehand canter to this.
So I agree that this probably has nothing to do with bucking, or even wanting to buck. It is falling onto the forehand as a strength or perhaps body sore issue. It is also possible that your horse is more on the forehand than you think most of the time, and the transitions just make this really obvious.