You're seeing powder being forced out of the top floors in the picture linked.
That's interesting, but the fire died down quickly after the explosion. Even Dilbert, my eternal 9/11 conspiracy foe, has agreed that the et fuel burned maybe 10 minutes. All the fire after that was office furniture that was required to meet fire code. What this means is that the same types of fires that have happened in every other steel reinforced building in history (none of which have ever collapsed due to fire) were what caused a collapse in a building designed to be hit by a plane and have a sever fire? That seems rather odd. In addition to that, the picture shows that the building did not give at the crash point, where one would assume the impact damage would have effected the strength of the building. It started collapsing from the top. Admittedly, this is difficult to see in the videos, but the picture linked above is from a video and when it's slowed down enough, one can see that before the point of impact gives, the top begins to collapse. This would assume that the heat at the tope of the building was harder on the building than a Boeing 747-200 crash and subsequent fire.

Again, this is counterintuitive.
The jet fuel burned for only a few minutes, at a temperature not only not hot enough to melt steel, but to even weaken it in any meaningful amount, and then all that was burning was paper, chairs, desks, carpet, etc. These all burn relatively cold. Not only that, but you're talking about heat from a small fire in only one place in the building spreading across an amazing amount of steel, and not just rising, but spreading in all directions. I would ask why you think that steel far away from and above the fire would be hotter than the steel at the fire.

I understand the idea of pancaking, or the possible domino effect of collapsing floors. I don't know if there's a reasonable explanation as to how the pancaking originated.

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.