the WTC was unlike any other building ever built, unlike the extremely strong, extremely heavy and space constrictive block type building ( squares of iron beams form the structure ) the WTC was built with a internal core support and a external structural support, with floors being suspended between each with steel trusses
so unlike a regular structure, there isn't much redundancy in the WTC structure, so that when the steel trusses' connections to the inside ( I believe the inside structures were compromised more than the outer structure, as that makes sense because the rebuttal to the ductile steel discounts nuts and bolts being sheared off and the internal structures being weakened by fire, which i find to be a glaring oversight in the rebuttal ) and outer structures was compromised ( situation only exascerbated by the burning fuel ) and collapsed from the inside.
it may have been a floor by floor event, each floor resting on the next until the structures were unable to cope with the stresses
then again you have one side of the building, with most of the load bearing structure being destroyed by the impact, you know, an enormous hole will weaken any structure, ESPECIALLY one with NO redundancy in the load bearing structures
{EDIT} that's my take on it.