Rust around the wheel arches usually starts due to the build up of dirt etc that holds moisture, its not the only thing that causes it but is a big contributing factor.
The foam front liners hold lots of moisture, I took them out of my Passat as they were like a wet sponge.
Its very hard to stop it once it has set in, a good clean now and again helps, and the use of a good quality wax treatment from the likes of dinitrol or bilt hamber sprayed around the arches every year or so helps keep it at bay.
