This can happen as a result of a few things:
-Your Xbox 360 is switched from vertical to horizontal position during gameplay
-You trip and make your console tip over or move in anyway.
"Let me just set up the audio/video cords for my Xbox"
*Suddenly trips over the 360. A strange whirring sound begins and the console shakes*
"Oh crap. Its the 360 Death-Scratch (360DS) "
*You take the CD out to see that a dangerously beautiful scratch now runs the perimeter of the disc. The following morning you commit suicide.*