Once again, looking at wikipedia, there's an explanation.
"Total progressive collapse
The collapse of the World Trade Center has been called "the most infamous paradigm" of progressive collapse.[51] Once the collapse initiated, the mass of failing floors overwhelmed the floors below, causing a progressive series of floor failures which accelerated as the sequence progressed. Soon large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside pushed by the increasing pile of rubble. The result was the walls peeling off and separating away from the buildings by a large distance (about 500 feet in some cases), hitting other neighboring buildings. Some connections broke as the bolts snapped, leaving many panels randomly scattered.[52] Significant parts of the naked cores (about 60 stories for the North Tower and 40 for the South Tower) remained standing for some seconds before they also collapsed themselves.[12]"
http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center
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