Technically, you are more likely to fart at altitude than when you descend back to earth!
As the cabin climbs to 8000 ft altitude, the gasses in your stomach expand, causing pressure to increase in the confined stomach. This needs releasing somehow, and there are only two places for it to escape from, the easiest results in farting. [Why do you think that cabin crew walk quickly down the cabin, leaving an odour lingering
]
Descending back to earth works in the opposite direction.