I would get the farrier, chairo, and dentist out just because his back might be sore but his teeth could be sore as well and when you apply pressure to the mouth (turning him) he feels the pain and the only way of letting you know he is sore is by bucking. Or if you have recently bought him he could be testing you which he will eventually grow out of. I hope this helped. :)
I would recommend checking his back - if a horse has been going really well and then all of a sudden starts bucking it can usually be soreness in their back muscles. Also check if you saddle fits and isn't pinching because if he is slowly changing shape with the constant work you saddle might not be fitting him as well as you originally thought.