I'm sure that it would be automatic if set like this but are you sure you havent got the power saving settings set up within the properties? I'm pretty sure there is one for hard drive shutdown after x amount of hours or something. It's within the screensaver options I think, when you right click on the desktop and go to properties. Might be on a different tab other than screensavers but it's somewhere around there. Check them (wild guess but better to be safe than sorry).
But indeed, noticed any patterns in time? I.e. after x amount of minutes/hours of being idle, same time every day etc?