What is their motivation for wanting to take down the wall? It's got to be more than a grudge Mance has against the Nights watch.
So why do they attack the Nights watch?
i would think that many of the wildlings would like to move south of the wall to live more comfortably during the coming winter. the wall (and the watch) represents a man-made barrier that prevents freemen from moving/migrating where they wish. of course, with the Others in mind it would also be a good reason to leave the area north of the Wall.
on the back of ASOS book (at least the paperback edition) it states, "...an army of barbaric wildlings arrives from the outermost limits of civilization, accompanied by a horde of mythical Others..."
however, from what i've gathered, they are not working together. early in this book there is a reason that clearly leads me to think this. but, i'm unsure even with it being early in the book that i should state it without spoiler tags. so...