I'm not sure why they are treating your PFD like a dispute but I can answer the "why" as to them just now, suddenly, sending you letters...it is because this is what they do. It is exactly what they did to me and several others that I've read about on here.
You never hear from them at all..no dunning..nothing. You send THEM something (in my case, a DV). They don't reply and then all of the sudden Capital Management/Resurgent/Sherman (et al) is sending a dunning. Coincidental timing? NOT lol
You could go around and around trying to get them to answer you (you definitely don't need them reporting "customer disputes" on your CRs when you didn't). OR, you could go straight to the BBB and file a complaint. If you never heard from them until this dunning after YOU contacting them first..you have a valid complaint. They should have sent you a dunning within 5 days of reporting on your CRs. I'd file the complaint based on FDCPA violations. Ask them to verify or delete.
Trying to negotiate a PFD with them will drive you mad because they will just take you in circles. Trust me.
Starting Score: EQ = 550; TU = 491(2/10/12)