Don't know what causes it but I have the same situation on Motorcraft double platinum plugs I removed after 60,000 miles from my 1996 Ford Zetec 2L. They were running fine and really didn't have to be changed and not that worn. I didn't know this till I took them out. I had bought new plugs already so what the heck I changed them as 60,000 miles is the recommended mileage for changing plugs in this engine.
Whimsey