Any common factors, such as how long you've been using the machine, whether you're using the keyboard or the mouse at the time, etc?
If music is playing at the time, does it keep going, or does it stop to? That is to say, is the display simply turning off, or is the machine going to sleep?
I'm thinking perhaps you've got a "hot corner" set up - try moving the pointer right into each of the four corners of your display, see if you can trigger it like that. If so, take a look under the Mission Control icon in System Preferences.
Alternatively, it could be a faulty power button!