[Porsche-O-Phile]

Low pressure is created over the fuselage which tends to want to suck the doors open. Opening them is easy. Closing them once they've popped, not so much. Airplane doors sometimes have an additional "catch" to keep the doors from opening too far and theoretically catching anyone/anything from falling out, but they're not foolproof. Most Cessnas I've seen only have one latch and if it pops you usually have to wait until landing to get it all the way shut again. Very difficult to re-seat them in flight.