Sounds like normal surface cracking on a rotor that's really been used. cross drilled, slotted, non featured rotors will all suffer from that if they are run to full temp and cooled repeatedly in a racing scenario.
Later
"Only two things are infinite, the universe and human stupidity, and I'm not sure about the universe." -Albert Einstein
When was the last time you *really* did something with your Z to call yourself a Z enthusiast?