I had a 4 year old gelding that did this too.
Same thing he was real laid back and lazy.
The trainer had me lunge him and then we worked him on the rail at the trot. Every time he cut a corner or attempted to buck she had me use the outside rein and put him into the wall. A couple of times like that he knocked it off. We moved up to the lope and did it the same way.
After 3 weeks of this he was fine-even on the trail. If I felt he was going to try it, I would just disengage his hindquarters.
He turned out to be the best horse I ever owned, sad thing is he died 5 months later.