Possibly wrong but ill take a guess, since the rockman rotor will be light and light mostly = weak, then it will be bendable/crack easy. And if you look at the bb7, (at least in mine) when you pull it in, the rotor will put to one side/ flex. This rotor has possibly been to weak to withstand this flex, and has cracked.
Should be normal though, everything cracks at some point
Hope i helped you mate