The type of aircraft involved has a rear ramp that I believe can be opened in flight. Occam's Razor suggests to me the most likely reason for this sad occurrence is the first officer opening the ramp and attempting to view the landing gear, leading to an accidental fall from the aircraft.

In USAF loadmaster training, we were instructed never to approach an open door or ramp in flight without either a harness or a parachute. No idea if this sort of gear was available to this crew.