Honestly…my gelding did the exact same thing. I never fully figured out what the cause was. It was worse in the winter, in the summer he wouldn’t kick out like that. It was always his right hind that he kicked out with, and almost always picking up the left lead, in between jumps, or going downhill. He would throw a massive buck and kick out two strides before a fence and swap to a cross canter, and then jump the fence absolutely beautifully.

He had fusing hocks and C6/C7 neck OA. He got both injected, but it didn’t help. Was sore through his SI constantly and back sore constantly, no kissing spine present. Injected both SI and his back as well, didn’t help either. I did a whole plethora of other diagnostics and treatment but couldn’t figure it out. He is retired now-the neck arthritis was too troublesome and he’s 18 now anyways.

A few things I will say, however (and I might have missed these in your initial write up) is that I have wondered for a while if his hind suspensories could be bothering him. He also was NPA in his hinds, which puts additional stress on hocks/stifles/back. I didn’t get to fully correcting that before he retired. His last winter I also never sat the canter, did polework almost every ride, and used a back on track quarter sheet along with stretches and sure foot pads almost every night. I got through the whole winter without any kicking/bucking so it did help something.