Tunneling is the word I think. The voltage goes from the center straight down into the shaft instead of passing through the built in resistor. All forms of excess resistance including worn plugs, bad wires, and damaged internal resister probably increase chances of such a failure. Many that I have seen have also been POS aftermarket ones. I would throw away any rotor that is black. All Bosch rotors are brown.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician