Why cars rust

A few years ago, I had a 67 Belvedere shipped out from Oregon. I paid $750 for the car, so it was no show piece. When I picked it up, It was totally rust free, in the trunk, under the hood, bumpers were still nice... This car had spent the last few years under a pine tree, and had about 4" of wet pine needles stuck all over it, it had obviously not been pampered. All I did was take it to a car wash, (still on the trailer I picked it up on), wash it and bring it home. I looked at it a week later, and the bumpers had started to rust! In the few years it has been stored here, it has developed numerous surface rust spots. Ths underside is still nice, as I oiled the snot out of it the day after I got it. This car has NEVER been driven out of the yard. I believe we get alot of acid rain here in Vermont.