I am guaranteeing that almost 90% of the time bucking will be caused from: 1) Ill fitting tack, tooth pain, lameness, or sore spots, or possibly if he had ever been abused he may associate running with pain, so by bucking he is trying to escape the pain he feels in his head. Or, 2) A horse that hasn't got a good foundation in training, or hasn't had to much time being trained, or has had a bad trainer. 3) THE RIDER. Most likely this will be the cause.
The remaining 10% may be simply the horse. My OTTB had a bad bucking problem, ugh, I tried everything but as soon as this horse started respecting me, and trusting me, the bucking disappeared. He had used his energy for bucking, now I give him a job when he has energy welling up in him like a fountain. I go over grids if I feel he is about to explode. Give your horse a JOB, don't just randomly run and lope through pastures all the time.
Anyhow, I am assuming its the RIDER.
If you are confusing him by telling him to run, by kicking him and unconsciously pulling back on the reins he will get confused. But IMO, most likely he needs to go back to the basics of training. I think you are rushing him to fast. Establish walking in lead, and trotting in lead, and then walking, trotting, loping in saddle until you have complete control over him, and then, and only then maybe attempt running.
You can't expect your horse to behave perfectly if his ground training basics arent in place.
Don't be the rider who gallops all night and never stops to think of their horse, or how THEY are riding.
My 2 cents anyway.