I have a AQHA gelding who at the age of 7 began windsucking after meals- it was ABSOLUTELY because of stomach discomfort. I had him scoped for ulcers, and sure enough he had them. The key is that he never did it until right after he ate- it was completely obvious that eating was causing discomfort, and sucking wind helped alleviate the pain. I have done a ton of research on the subject- studies are really supporting that many horses are not windsucking out of behavior issues but because it is giving them relief in the stomach. Unfortunately, it is an endorphin rush for them- so try to solve the stomach issues before it becomes a fun, feel good habit for them. Good luck!