This was very unusual behavior. The computer crashed. I was looking at an unprogressed progress bar with the word "Erase" or "Erasing" in it, and then Boom! My computer rebooted. No beach balls, no nothin'; white screen. I have a 10.9.3 mac mini with 8GB of memory running the same programs I always do: Safari, Arduino, Mail, Firefox and Terminal.
I was doing nothing else at the time of the crash, but Mail might have been processing an incoming message.