At a guess? The impact and fire from the aircraft weakened several floors below the point of impact. Eventually those floors gave way, at which point the already weakened tower had to cope with what amounted to a 15 story building being dropped on it from three stories up. Momentum of the top floors collapsed the rest of the tower.
An engineer? No, but I did stay in a Holiday Inn Express last night.
Amazing...watched it live...lived through it....and STILL, there are nutjobs trying to revise history.
I didn't stay in a Holiday Inn Express, but I am an architect (r-q-tek... get it?) and I can tell you that your assessment is correct.
As for my tag line... this issue *IS* settled because it *IS* science