I've had this occur on my DPE's (forged, 2 piece) wheels and cheaper wheels (axis, racinghart, etc.).
On the DPE's it was on the lip, DPE said they were clearcoated but they eventually got these pits. I ended up stripping the clearcoat down and had to religiously polish the lips to make them look good. In the end these expensive wheels caused me more grief and maintenance than regular wheels.
On the cheaper wheels it was also always on the lip, never on the spokes of the wheels. I noticed it more when it rained, I figured the sand/dirt/brake dust would swirl around and when parked would stay in that particular spot and start eating away at the finish.
On my Schnitzers, MVRs, and Forgestars I never had this issue and they didn't have lips just full face wheels with spokes going to the edge of the wheels.
BMW's are known for their excessive brake dust, I eventually got low dust pads but they didn't have the same "bite" as the oem pads.