If you smoke a hit, you may regurgitate your undigested food. Or you may cough and jolt your stomach muscles enough to do it. Cannibus doesn't induce vomiting on it's own, there must be a contributing factor. If it did then wouldn't everyone get sick the first time their stomach's experience weed-smoke? If you smoke it without regulating your breathing, you're bound to get air-bubbles you'll be squeezing out for a good while.