When your horse consumes too much food, water or air, her stomach can expand rapidly. Your horse's stomach is small relative it its overall digestive tract and she is unable to vomit. Severe overeating can cause the stomach to dilate excessively and rupture. Ruptures are fatal.
The most common cause of gastric ruptures is grain overload, but rapid eating (common in socially inferior herd members), vices like cribbing and wind sucking and overconsumption of cold water on a hot day may trigger dilation, too.