Primers are , by and large, very uniform. BUT we are dealing with a shock sensitive chemical mixture. And with any chemical mixture, even when dealing with the most stringent QC, there will be a very small percentage at both ends of the performance spectrum.
And, being what they are, primers that are tested have little appeal to the buying public.
Primers have been known (or believed) to have gone off from impact, shock, static electricity, and for no determined reason. I think being dropped is probably the top on the list, but know of no reliable statistics on the matter.
Its rare, but it does happen. Usually it is assumed that the primer was struck (if even lightly), but it might also just be shock. Eitherway, its very, very rare.
Not that I think it applies in this case. I think it is staged.
All else being equal (and it almost never is) bigger bullets tend to work better.