OK, well, I'll try to explain my reasoning a little more. Its a little hard to explain so bear with me :P
Lets start from the beginning.
1. Plane zooms in and impacts the building.
What happens?
A) Large parts of the outside of the building are destroyed. The building was constructed in such a way that the outside parts are a major part of the support system (main core, inside support steal, outside frame). Less some support the inside supports are now strained more than usual.
B) The plane continues through, stripping the steel inside of it's fire protection, leaving it directly exposed.
C) Flaming jet fuel is shot through multiple entire floors, coating the exposed steel and everything else inside with blazing death.
Misc) Further damage due to force of impact unclear.
2. Jet fuel burning on multiple floors
A) The fire is burning directly on exposed steel support beams. Heating them up VERY quickly. Parts of the steel expand as they are heated. Some parts connected to those beams may not heat as quickly. Expansion rates are now really out of whack further weakening the already weakened building.
B) The fire may also be effecting other parts of the support structure. Flames can be seen pouring out of the large holes further weakening the outer support frame.
C) Liquid fire pours into elevator shafts and some (now severed) stair wells. further spreading the heat.
D) Flames burn through to floors above, spreading the heat even more and weakening more areas.
E) Flame slowly dies out after a few minutes, leaving some areas of the steel support beams red hot, others not so hot. some parts are still at the same temperature.
Now these beams are all out of whack. Not only are they expanded in some areas and not in others, they are also now supporting extra weight from the parts of the outer frame that is now extremely damaged. Some parts of those beams are now starting to cool off while others are still blazing hot which further weakens the already delicately balanced structure. Its not just the heat that the steel beams have to deal with, they're holding up many tons of weight as well. Now these beams are dealing with both of those factors.
The force of impact itself could have shaken things enough to cause major structural flaws. Perhaps some of the rivets holding some beams in were popped right out? There's many many tons of stress on those rivets, and that was one powerful impact.
3. Added factors.
A) WIND - At that height the wind is always whipping and blowing like a mofo. Normally skyscrapers sway in the wind. That is a lot of added stress, and should not be taken lightly. No matter how calm it may seem at ground level it is a totally different story 100+ floors into the air.
B) (for the second tower to fall) Added stress from the first tower falling (obviously).
Alrighty.. So now we have a tower blasted into, outer support severely weakened.. inner supports also feeling the force of impact.. extremely hot fires directly on exposed metal supports that are already under extreme stresses due to weakened support in other areas.. the expansion and contraction of those stressed beams.. and on top of all that the wind causing the tower to sway. If you ask me thats one giant recipe for disaster.
Ok now for the collapse part.
http://www.youtube.com/watch?v=OOsQpJ1DzsQ When i look at that i see it buckling at the point of impact. Thats a nice angle.
When you look at the top floors before the collapse you see a lot of smoke pouring out, that is what is being flushed out during the collapse at the top. and when the building starts to compact and fall you see the outer concrete blasted out which rains down. You can see just looking at it that a large portion of it was just shaken and crushed into dust.
When you see video of the collapse at ground level you'll notice that everyone in the street is running from a MASSIVE debris cloud that is like 15-20 stories high and expands out over an extremely large area. Thats a lot of debris. so its not all sitting nice and neat in the footprint.
Well, that was a lot of rambling. I'm not sure what i accomplished there hehe. I hope it explains what i think was going on there tho.