Given that its unlikely we'll see an official incident report on this one, anyone care to speculate on how someone can fall out of one of these devices, my thought process looks something like this;
Possibly dynamic rollout, hardware had suffered "D-Ring reversal" and the stop/start at the fuselage caused the final rollout to occur - unlikely as the survivor doesn't appear to be in the device while falling
Rollout of only one side of the device - also unlikely as no "flap" appears to drop down when the survivor falls
Survivor seated too low in the vest - also unlikely as the survivor appears to fall out head first
My best guess is that the legs were put through the arm holes. But interested to hear others thoughts.