This is a shot in the dark, I know, but do you by chance participate in Google's Screenwise Trends program? Their app was causing my Nexus 6 to randomly reboot pretty much every day. Some days 2 or 3 times. I just dealt with it for a long time, not knowing what the cause was - I thought it was a bad update install as well. I uninstalled the app after I saw a review in which someone complained about their Nexus 6 rebooting at random after installation, and sure enough, that was the culprit for me as well. I haven't had any such problems with the app on other devices, and no more random reboots for my N6.