Does your case, bag, or anything you keep around the Mac have a magnet? Sleep is initiated by a magnet in the lid, so other magnets will mess with it too.
Probably a longshot, but I use the "hot corner" settings, where the screen turns off when moving the mouse pointer to a specific corner (to save battery if I know I won't need the screen for a few minutes). A friend borrowed it for a while, and he came back complaining that it randomly shut down the screen Maybe worth a check that it wasn't activated by accident. You change it it System Preferences -> Desktop & Screen Saver -> Screen Saver tab -> Hot Corners.