In this training we go over aaaaaaaaaaaall the reasons I have uncovered for the question, "why does my horse buck?" Hopefully that helps you to start down the trouble shooting road for how to work with your horse to undo the bucking issues.
I have a youtube training to accompany this podcast covering the horse that has topline issues and a posterior pelvic rotation that is exacerbating the buck issue. You can watch it here
If you want to check out how we go about building topline so that it helps the integrity of the horses spine to prevent bucking, check out our free training here: https://www.equestrianmovement.com/stopdrillingcircles