I think its just a side effect of repeated hot cold cycles on a piece of steel.
Iv seen it occur in other totaly differnt types of machinary that under goes repeat heat cycling. Hydrolic rams for example.
And of course we all know that rapid a rapid heat change can do bad things to metals, for example its a good way to shatter as casting.
Something as simple as heavy brake use, then driving into a puddle could easily cause that.
However, I was only offering further arguments against the idea put forward in the link provided by Curtis, which in all reality sounds deeply floored and lacks and understanding of what happens.
One thing should however be pointed out, there are systems in use today which will in theory never wear out. For example the brakes on the Lotus Elise in theory never wear out the pads, or the discs. The idea being material is transfered between the pad and disc during use. So it is possible for material transfer occur, however they are both made from the same Aluminuim based material. Very unlike a converional disc and pad which are made from very dis-similar materials.