We have had that problem here, as well. I have never been able to track it down.
I will say that it only occurs to our users with poor workflow (media on desktop, media scattered, bad sequence management, needlessly long sequences), and that almost always (and perhaps just "always"

, we're able to recover to an autosave version from not too far in the past.
Someone from LAFCPUG recommended manually saving to a new project once or twice a day, and I adopted that. That way you're never more than 4 hours from a re-start, even if the worst was to happen. I keep mine on a thumbdrive. Furthermore, when I'm working on a big project, I do a Media Managed version once or twice a week to an external hard drive. Cheap insurance, IMHO.