Windows 7: Unexpected soft shutdown, no error, not abortable

I just had a shutdown that was indiscernible from a normal shutdown. I did not initialize it.
When I saw the telltale screen darkening, I quickly opened cmd and ran shutdown -a, but was told no shutdown was in progress. Seconds later, the shutdown continued. There are no abnormal messages in eventvwr.
I just scanned the system for general malware and rootkits, and couldn't find either.
Any ideas what is going on? Any logs I can upload for you?