Considered that, but checked in the Event Log and the only update applied was to my AV program (the MS free one) about 3 hours before. WIndows Updates necessitating reboots all show log entries there. I have real-time scanning disabled during the testing, but allowed the program itself to remain in memory to prevent any instability caused by low-level drivers being missing. Also interesting that I apparently didn't disable system scans, and it ran a manual scan during the overclock test without causing any hiccups.
That being said, you could still be entirely correct. I both don't know if MS Essentials causes reboots like that (as this is my first time using it) and have only eliminated the most common source of reboots via looking in the log. Any third-party reboot would not show there, although it would show a 'system shutdown' message that was not present, indicating unexpected reboot.
The absence of a serious error message or a bug check event tells me it wasn't a blue screen, but it still was unexpected. It could even have been a power fluctuation, although those don't normally cause reboots, just pure power offs.