Pro just soft reset itself

[CaliforniaPlaya]

Either it soft reset or turned on without me knowing it was turned off. It was sitting on a wood desk, I hadn't touched it for at least four hours and the last time I did use it, I left it on with the battery close to 80% and it wasn't on the charger. I heard a short vibrate and thought it was going to ring (I've got it set to ring and vibrate and it usually starts to vibrate a fraction of a second before it audibly rings), when I didn't hear anything else, I picked it up to see why it was vibrating and see the boot screen. I kept watching it and it did complete powering up and appears to be working. The battery level is still close to where it was the last time I used it at three full bars. It didn't move or fall or get bumped or anything like that, nobody was close to it and hadn't been for many minutes. Has anyone else ever seen the Pro soft reset on its own?

Thanks!!!