My 9 yr old gelding has started ducking barrels suddenly. What has been the reason horses have done this for others and how did you fix it?

Horse that normally work, just doesn't stop working for no reason.  I won't take him to a good performance vet and do a lameness evaluation.

Ditto to response above. If a horse starts doing something suddenly and out of character, I would go to a performance vet for a lameness eval. A lot of times, something is hurting and the only way the horse knows to "get out of it" is to avoid their job.

Mine have always had  a vet issue when this happens. Sometimes it's as simple as injections. It has also been a torn suspensory in one case and pedal osteitis in another. I truely believe good horses don't just stop working for no reason..

My bet would be bleeding. I didn't want to believe it with mine and wasted a whole season trying to fix it with training and riding. A few ccs of Lasix fixed it right away. 

Bleeding, hocks, stifles, kissing spine can all do it. 99.9% of the time, I would say 100% but theres always gonna be one weirdo that doesn't fit, its because they hurt. Figure out where and go from there