We often RDP into our servers. Sometimes if we end the RDP session via Start, Shutdown, Log-off, the server restarts.
It seems to happen when the server is waiting for a report due to a patch pushed out via WSUS, but we may be waiting for a maintainence window before rebooting.
Nothing quite like the cold-sweat when you notice that your production SQL Server or Exchange mail store is rebooting in the middle of the day!
Our work around is to just disconnect, not log off, but this causes issues with the next user who needs to log in.