seizure ?
Something is wrong with the motor that makes it get real hot... the aluminum piston grows larger with the heat... till its too tight in the cylinder... then it starts to seize (stop moving)... the piston gets a big 'bruise' on it... with scrapes and scuffing.
Then you get to fix it.
This is why you do a plug chop every once in a while... to see how much heat you are getting.