[gordner]

HALO jumps occur from extreme altitudes sometimes from aircraft that don't pressurize the cabin area and some after in flight controlled decompression. At that point opening a door is not a particularly dangerous task, though windy as hell.
If you look at a door on a pressurized aircraft at say a 10 PSI differential, say the door is 9 square feet, every square foot is 144 square inches, each inch experiences an outward pressure of 10 lbs, that means the door as a whole is stressed to the tune of 12 960 lbs