I have caught my Nexus rebooting 5 occasions in the last week and there is absolutely no factor I can see about what it was doing just before it. It is not updates, or any app, and it did it twice while the "sleep" function was going with no apps in the recently used area. It is not a problem, well, one of those times it rebooted 4 times continuously all by itself and then came up normal. It is not rooted, only a few apps have been added, no screensaver apps but a weather app is on it. I wondered if it was "SeekDroid" which I loaded but have not configured yet? I am getting great battery life and have no complaints about the phone, but wanted to mention in this thread that mine reboots as well.
Oh, and from day one I have had it set not to automatically update anything, not the apps or OS or Google+ so it is not like it needs to reset to let a running app restart.