Probably just a defect in coating. Bummer it showed itself that late out of warranty :( My Ranger had a similar problem. The bottom seam of the passenger door (where the inner met the outer) started rusting after 2 years (about 20,000 miles). It was somewhat fixed under warranty, but it came back about 6 years later. It was the only panel to rust on the truck for 12 years (stored outside all the time, driven in winter, etc..).