It appears it can be a result of "flex" of the slide on the frame. My poor choice of words, not the internet's, but there's some pretty good YT videos showing 100% reliable failures. The videos aren't done by range bozos, but by folks who are following the same methodology. Essentially, as the gun is pinched or shifted in a holster, it's causing the slide/receiver interface to shift enough that it releases the striker.
What's also interesting is that not every gun does it, but certain lots which leads many to believe the QA process isn't the greatest (from Sig? No way!).
I've actually only had a very cursory interest in the whole thing (I don't own one) and was a doubter for a long time, but ended up seeing some respected sources reproducing the issue and have become more of a believer.