I read this thread an hour ago, but haven't been able to get it out of my mind since.
I'm hardly a gunsmith, but cannot logically figure out how a striker-fired Glock can fire without having the trigger go through its normal cycle.
That being said, I have a Serpa holster that I only wear when working in the yard. I've been looking at it for the past hour and can't see any way that the trigger can get pulled. The OP's discharge has scared the heck outta me, and my Serpa is staying in the drawer until we hear an answer that explains the issue.
G'day and G'lock.