Lots of cars that rust in that spot do it from the inside out. Some of it is the body sealer/foam the factory uses that retain moisture (I think that is the most common cause). Sad really

 

2 reasons IMO:
-In some parts of the country with heavy winter salt, there is nothing you can do no matter what car it is. It's just impossible to totally clean that salt off, and once it starts to rust, it'll just keep spreading. With these cars getting older, it's more likely to be worse from time.
-The w body in general has a design issue with that area and rust. They've been having that issue for 20+ years now. The newer ones seem a bit better, but I've seen it on all years back when I lived in the Midwest.

 

I think it has something to do with gas leaking down right in the hold that holds the "gas cap string", speeding up the erosion/corrosion process..

 

That may help speed it, but I can say for sure it isn't required for the rust to form. I've never dripped a drop of gas outside of the filler neck hole on my monte and I have a pretty significant rusting patch below it.

 

I disagree with this. Yes there can be rust formation along the horizontal joint where the quarter panel and bumper meet (that's where any gas that leaks would go) but the main area of concern here is the rear quarters in the rear wheel arch lips which (A) seems to be toward the front of the car and (B) worse on the passenger side. A lot of these cars have rust in the rear passenger lip but not the rear driver lip - mine included. There has to be something different with that side that's common on all of these cars and it could be the way that side was assembled at the factory - could be the foam, could be how the pinch welds were done, etc.

 

My 2006 was also eaten from the inside out, and the lip of the fender was completely gone by 2014. Had this looked at by a dealership in 2010 or so, but they wouldn't do anything because it was just surface rust at the time. Now that I'm trying to bring it back up to tip-top shape, I'm mad at myself for not pressing the issue with them...

 

when I got into a accident with mine had both sides fixed before it started that the shop I took it to. did a liner type of under coating with it and didn't have that problem yet but to be honest it is sad on some quality issues with there vehicles.