The neatest package would be a combination of the force of the explosion with a local phenomenon. In "Yesterday's Enterprise
" it also appeared that mere fierce firefight could open a temporal rift - but perhaps only under specific rare circumstances that cannot be easily reproduced or even identified?
One'd think there would be studies afterward. But one'd think there would have been studies back when the Bozeman
disappeared already (just three weeks away from civilization, so basically in an area that would have to be secured against future accidents of the sort). And they seem to have come up with nothing, as our E-D heroes had no archived speculation to build on.