air - Reason for strange "canyons" of dust on the cooling platform

I took apart my cooling platform Something like this :

And I found something like this inside :

I wonder why around the hollows are the "canyon like" structures.

PS: I also drop this platform on the floor, maybe this could be a reason ?

EDIT :

Everybody seems (except Brandon Enright answer) to ignore the word "dust" in this question. Those structures are not in the plastic itself. It is dust ON the plastic.

Proof :

Completely wiped out with my finger :

Answer

I've seen this too. The circles that the "dust canyons" are around are marks from the injection molding process. Specifically, they're the ejector pin marks made when the pins push the part out of the mold. The force of pushing the part out probably creates radial stress cracks in the grain of the plastic. If you use a microscope to look at the surface of the plastic you'll probably see a rougher patch in the shape of those canyon lines and that's what dust is clinging on to.

There are a lot of different types of injection molding defects. Splay marks are a similar feature of injection molding that could look like your image.