Well, my theory of what happened is:
Terezi had Disk 2, and put The Scratch on it -- albeit physically -- and ultimately caused the game to begin destroying itself from the inside. This would trigger the fail-safe function of a hard reset, but under a VERY slim circumstance, the disk had been scratched in the data packet(s) that CAUSED the hard reset function.
So basically, there's nothing standing in the way of the disk tearing apart reality.