How could a Ball Turret gunner fall out of a Ball turret?

Hello To All
 I have been doing research on my Uncle's service with the 384th BG and ran across Mission 57 on their Awesome Website.
On B17 A/C # 42-31495 . It say's that the Ball turret gunner fell through the ball turret at 12,000 ft. with no Parachute.
 How could this happen? Who locks the door on the ballturret?
Semper Fi
Phil

The gunner locks the door after entering the turret. The entrance door is also the escape hatch. However it wasn't used much for that purpose because the gunners parachute was in the aircraft, not in the turret, due to lack of space. He also had a safety belt around his back to prevent just such an accident should the door accidently fall off. On one of our training flights the ball turret door fell off in flight but our gunner had his safety belt hooked and that prevented his falling out of the turret. He was pretty wind blown when the crew got him back in the aircraft. The experience did not dampen his interest in that position. He really liked being down there.

Bill has it pretty well pegged.
Of course if he had either failed to hook his back belt, or opened it on purpose, he could fall out should the door be blown off or otherwise opened.  I had heard of gunners who opened their door to attempt to get access to clear a jammed gun, but that seems foolish in the extreme.
I not only couldn't take a parachute with me into the ball, I couldn't take a flak helmet or flak vest.  Very tight quarters.

Makes my hands sweat just thinking about being in the ball turret!