I can appreciate that dirt or debris behind the rotor would cause a problem, but how would this happen BEFORE the rotors were replaced?

I did ask to have the runout of the rotors measured during my last phone call. I agree with this advice and inspection.

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1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)