What is Shutdown Cause -108? My rMBP 15" just shut down by itself when idling, screen turned off.

When I rebooted it, I checked the Console to see why it shut down, and found this message: 10/27/12 1:21:36.000 AM kernel[0]: Previous Shutdown Cause: -108

What is Shutdown Cause -108?
According to this page, Shutdown Cause -108 seems to be due to a lack of memory.