I have a hard time believing there is enough inertia to set off the primer. I'm one of those reloaders that just HAS to know what it takes to set off a primer. It actually takes quite a bit of impact to set one off - even set into a bench vise and with a punch used to simulate the firing pin. (Yes, goggles, gloves, muffs, safety equipment of all sorts was employed in my experimentations) That's why I asked about the hammer drop. But your explanation is the only logical answer, so Occam says you must be right.
