Spontaneous restart -- causes?

My Roamio basic spontaneously restarted today at 10:52 am ET while I was watching a recording. I could find no apparent reason. There was no software update and the Tuning Adapter did NOT reboot. It's on a UPS.

The box is almost two years old. I can think of only two possible causes: (1) HDD failing or (2) just a software glitch. My guide data connection and GC times look OK.

Any other ideas?

Spontaneous restart -- causes?