I'm assuming the Detroit Police have the holster and weapon as evidence. The only thing I can imagine is that she was down low behind him (engaged in some of the a fore mentioned bumping and grinding) and she tilted the butt of the gun forward and depressed the trigger.
I've seen a couple of striker fired guns malfunction (almost always due to an amateur gunsmith and a dremel) but I've seen more NDs. With Glock, M&Ps and XDs I always assume human error first.
