I had my first reboot today and managed to recreate it once.
It happened as I was leaning in the doorway to our server farm at work. The room is a Faraday cage so there's no signal inside.
As I crossed into the room, I felt the phone vibrate, took it out of my pocket to see it rebooting.
When it was back up, I tried moving in and out of the room and on the second attempt, it rebooted again. I couldn't get it to do it a third time.
Where I live, the signal (3 UK) is very strong except in our server farm. I wonder if it's related to the way the OS handles a loss of signal or when something tries to update when there's no signal.