Adrian of the UK wrote:
If the the top five floors give up and crash down into the 105 floors below, then there is resistance because the top five floors are gravitiy assisted, so in your mind they have the potential energy to crash through the floors below! But the 110 floors below are also assisted by gravity. The (cold undamaged steel structure) foundations pushing into the ground - they resist pressure and the 110 floors already pinned to the floor have a greater resistance to the potential energy of 5 flaming floors above! That is why they should not free fall like they did, it's never happened to a building like this before and never will again in this way.

Consider anything material or organic, when it is wounded it recoils and closes around the point of attack or damage (a tree when it is felled, a knife wound to a human, even a wrecking ball to a building).

Also it is possible that the explosives could've been engineered into the structure when it was first erected some thirty years ago! If the New American Century was conceived before the towers were built then it is entirely believable that this could have been a plan many years in the making.