What would have caused my Xbox One to turn all the way off?

I was playing Halo 3 earlier today when I told my Xbox One to turn itself off. Usually, it just goes to sleep, but then I heard it actually powering down a few minutes later. I didn't think of anything at first, but later this afternoon when I wanted to resume, I kept saying "Xbox on. Xbox on. Xbox on." It wouldn't turn on, so I had to turn it on with my controller. Sure enough, it had to boot up. I had to start the game up again, and unfortunately, I lost all progress in the mission that I was working on.

The only time I've noticed it shutting down completely by itself is when it was applying an update.

Ahh. That must have been it. Sucked, too, cuz I was right at the end of the mission I was on when it shut down.

Personally I only trust the resume as a convenience and not like a game save. There's too much that can happen to kill it.

Yeah, I should start saving more. Also, doesn't the Xbox One shut it self completely off after a matter of days?

I've never had that happen with my Xbox One.