What causes flatulence after going high in mountains but not planes?

Pressure. Airplane cabins pressurized - 8000ft above sea level to prevent hypoxia. High in the mountains could be significantly higher than that therefore less pressure and more expansion of intestinal gas. However, this would only apply to gas that was created prior to ascent. If you are ascending over a few days, this explanation won't hold and i believe you are somehow swallowing more air - ? Exertion?