We fart more when we fly. It's true -- and it has to do with the law of physics known as the ideal gas law.
High-altitude flatulence has to do with a change in atmospheric pressure and altitude. Airplane cabins are pressurized to be the equivalent to about an 8,000-feet altitude (which is a cruising altitude of about 565 mmHg) -- a significant pressure change for the human body, considering the pressure we're used to living on the ground is 760 mmHg. When the atmospheric pressure in the cabin decreases, the volume of intestinal gas in your bowels increases, and a greater volume of intestinal gas means you're going to pass more of it.
Remember: Half of the air in the airplane cabin is recirculated air, so break wind at the risk of your own (and your seatmate's) nose.