I would hazzard a guess and say they divert them there because it isn't a high population area.
7777 Military, Interceptor
7700 Emergency
7600 Lost communications
7500 Hijack
1200 Visual flight rules
Now, if your radar controller told you to switch to something like 1523, it is rather easy to hit 7500 during the switching process. The pilot can only switch one digit at time on most transponders.