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.
If the busses were the target, a motion activated switch or a timer, If the crowd (but not himself), a timer or remotely detonated device would be effective. For a suicide bombing, a mechanically closed switch (command detonated or 'deadman') would probably be the ideal.
Remotely activated detonators would have applications, too, but could be set off on some dupe to eliminate the basic witness.
A lot would depend on whether the bomber was working alone, whether he was the top guy or a "soldier" and the target.
I imagine any unexploded ordnance recovered will help answer those questions and help discern his intent, place in the pecking order, likelihood of working alone, etc.
It is no doubt being thoroughly studied while we speculate.
I just re-read your post. A deadman switch wired in without holding the plunger down would be a definite oops.
You would think the guy would have a manual arm/disarm switch, but you never know.
The scenario you described is a possibility, too.
It only takes a second of stupidity (not that the whole scenario isn't stupid) to end a lifetime.
You were lucky you weren't burned badly, imo...