Even the angle the device was held at could be deadly with some types of triggers if the switch were inadvertently turned on. He would not have had to be removing it, only jostled the contents of the pack wrong and turned the switch on on one of the devices.
I had a different theory.
I figure his design called for him to jack in a plunger switch. If he wired it improperly with a hot battery already connected that did not have a separate disconnect that he could switch on AFTER he jacked in the plunger, then as soon as he inserted the plunger jack the circuit would make momentarily until the jack was fully seated.
I have actually made this mistake myself when constructing a remote wired launch trigger for model rocketry. And hell, I'm a electrician by trade.
The rocket almost toasted me....LOL This is why you energize the power last in a two step sequence.
He may have made the same stupid mistake in the effort to make a simplistic detonator.. Just a theory based largely on a crapload of assumptions, but quite possible.