We had the same problem last year with our E-450, pad material flaking off in chunks. Asked the mechanic if he knew why, but said he could only speculate, age, water/salt, heat. He said that driving regularly/weekly MIGHT help, but do you really want to drive it in our winters with the salt.
Beverley and Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.