Anything DU says urks me :)
I do know floors came down first. Full floors just came collapsing down into the lobby before the building structure.
The 'pancake' theory seems believable to me. Once a single floor failed and tons of concrete pancaked down upon the floor below, it set off an unstoppable avalanche.
Floors can't collapse inside the outer building structure because it's all one and the same!! Each floor of a high rise building is essentually all one piece supported by pilars, stack one on top of the other. Some pillars are lessor supporting pillars that don't run through several levels, others are main supports which run through several levels to which the slab is attached to.
The outside of the building is merely sheets of glass that are hung and supported between each of these slabs.