The first time this happened, I chalked it up to coincidence. The second time was worrisome, and the third event was a disturbing pattern. I am not quite sure if this is an issue with the holster or the gun, however, this does not seem to be a common issue with the M&P and I have had no trouble with holsters of other makes (Bianchi, Fobus, serpa-retention style, etc). Neither has Heather, who also carries an M&P. That leads me to suspect the holster.
The best theory I've come up with is that the mag release is getting pushed in by the holster when I sit or wear it. The kydex portion of the holster does not cover the mag release. Moreover the leather near the mag release is scuffed slightly.
I am not giving up on the supertuck. As an immediate measure I've started checking the mag to make sure it is securely seated. I've also thought about the whole "tap rack bang" malfunction clearance issue. I'm going to change where I carry the supertuck and see if changing the position from 5:00 to 3:00 or 4:00 mitigates the issue. I can also change the height at which the holster rides which may help.
Still, a trend of the mag release getting hit which pops a mag out, which would induce a malfunction if I had to fire the pistol, is very problematic.