The Shockwave Theory
Attempts to explain how destruction can outrace falling rubble.
The gigantic impact forces
caused by the huge mass of the falling structure
landing on the floors below
travelled down the columns like a shockwave
faster than the entire structure fell.
The clouds of debris coming from the tower,
several storeys below the huge falling mass, probably result from
the sudden and almost explosive failure of each floor,
caused by the "shockwave".
-- University of Sydney