Are you bragging, asking for a fix or looking for a likely cause?
My first guess was a crack in the filler under the paint. If there is not a thick layer of bondo, I would agree with the back side impact or worry about some rust.
The usual cause of paint delaminations...poor surface preparation. IE, leaving a bit of dust behind, or touching a prepped surface with your fingers and leaving oil behind, etc. The only solution is to take it down to the surface where the delamination happened then re-prep and re-clean, primer and blend and match the remaining paint. A good paint guy can do it no problem, no clue how much it would cost. You can do it yourself, but blending and matching existing paint that has been weathering for who knows how long is something of an art.