Do you have any automatic update script running, like cron-apt?
Also, I've Seen this kind of behavior in Citrx Farms on some of my HP servers. The shutdown could come from an external source, like ILO.
Hmm, interesting, but It's not for me. Shutting down only one VPS under KVM (another two is good). I have no cron-apt or others, but the server was updated manually and from wheezy-backports, but just after this malfunction.