Probably rust buildup from not much use. That said, I have not seen good luck with that particular brand either. Rears look ugly too. If not drove much the hard use of the brakes a few times once a month will greatly reduce the rust buildup. You will never use them hard enough to hurt them. 99% of brake issues I see are rust, not hard use. .....not sure where u are but from rust on back pics, it will be a problem.