Whenever two or more army books have contradicting fluff they should have the writers fight it out to the death and whoever is left standing with the still beating heart of his opponent(s), his books fluff is considered cannon.

But I also believ that it was a combonation of both. Slann tried to orderly re align the mountains, Skaven just so happen to be using an extremly potent magical device in the vacinity and the resulting effect of the two magics mixing (Which is generally a bad idea unless you are Tzeentch) is the Dwarfs's lives getting alot more complicated.