I do have it set to auto update and applied a system update a week or two ago. That usually isn’t without a following period of instability while other apps get updated to match.
The typical occurrence was just “drop in pocket, walk a few hundred feet, take out phone, no response”. It would go through the usual boot up procedure after the long press, sometimes taking a little longer on the static screen before going to the fancy animated booting graphics. I cleared the system cache partition as a triage step but it happened a few more times afterward that afternoon. I also cleared the app drawer/task switcher history and manually rebooted, to the same effect.
Shutting down in my pocket seems unlikely because I would have to have held the power button for ~1 second, not noticed that it had vibrated when the flyout menu came up, and I would have had to hit the power down option instead of anywhere else on the screen. Not impossible, but a lot of silly things would have to happen repeatedly for it to be an ongoing issue. That said, I’ve been known to be silly and oblivious on more than one occasion. =)
Thanks again for your feedback!