Came out today and noticed a large white spot on my driver's door. Appears that the paint somehow oxidized? It definitely wasn't there when we did a photoshoot a few days ago. The car was sitting outside, it rained once or twice, and I wiped the car dry. Something similar happened about ten years ago on the rear fender and I had to repaint it. The car was been repainted at some point in the 90's. Rubbing with cutting compound is making it slightly better.

Any ideas what would cause this to happen?

 

Age...