Gas. Narrow esophagus. I think there's more but I don't remember where I read about it, so I'll have to do some hunting, it's been a few years. Mazy cat had projectile vomiting with her first $400 hair ball when she was about 18 months old. The fur had lodged where the stomach empties into the intestines and her stomach had filled with gas. Every time she tried to eat it would come back up...shooting almost across the room. X rays showed a belly full of gas.