Maybe it is the new style roadsalt, it (allegedly) has something in (animal fat? mollasses?) that makes it stick to the road for longer. Unfortunately it also sticks to windscreens, motorcycle helmet visors and (probably) brake disks.
I think that has a lot to do with the general car corrosion, but I don't think explains this scenario 
Again, why not the whole disc? And just the inboard outer edge.
Also, as mentioned earlier, this affects the pads as well, which wear away in the adjacent area. IMO its the rust that swells the metal and takes more pad material away than the non rusty area.
First stage - shiny new metal, new pads. All good
Second stage - light rust forms overnight. Gets swept off first application of brakes. No harm done.
Third stage - rust forms pits in the surface. Brakes sweep the surface rust off but the pits remain
Fourth stage - pits in disk surface cause accelerated wear of brake pad. Wear is uneven and pad edges wear first
Final stage - parts of the disc that rusted first (eg the edges) are allowed to rust in peace forming the big flakey rust
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Topic: Why do inner discs corrode? (Read 4399 times)