What would be even more interesting is the question why Chaos Dwarfs?
Why not Chaos Goblins, Chaos Orcs, Chaos Halflings, etc. etc.?
My guess is that Dwarfs in the Norse legends and folk beliefs are morally ambivalent, and many are outright evil. Even Tolkien makes a mention that in ME some Dwarfs were so consumed with envy that they became evil. So evil Dwarfs. Then GW made the obvious link to good Dwarfish armour, maybe even Dwarfish Chaos armour, and machines. Add an (un)healthy dosis of Punk, Giger and Cthulhu, and there you have it, I'd guess.
-Z