The miles of sediments that lay above the granite and greenstone of the Blue Ridge crest were washed away. But granite and greenstone are more resistant than the sedimentary rocks, and when they were exposed the erosion slowed. To the west, the softer carbonate rocks had been pushed downward by the buckling of the land that shaped these mountains. The limestone eroded rapidly, forming what is now the Shenandoah Valley. Erosion of the Blue Ridge speeded up during the ice ages when, most likely, the talus slopes that we now see along the western slopes of the mountain were formed.
Erosion is slower now, but it continues nevertheless.
The earth has a solid outer crust maybe fifty miles thick, floating on hot plastic material. But it's not a liquid. It can flow very slowly, in response to great pressure. A sideways pressure can make the crust buckle, so that it rises in some places and sinks in others. That's a simplified version of one theory.