It is my understanding that the twin towers were built around extremely strong cental core stuctures. Essentially, when the over-heated steel in the core started to lose its integrity (the strength of steel changes with "heat treating" - when heated to a sufficient temperature for enough time it begins to anneal, reducing its load carrying capability) the vertical supports failed. Before the the structure tipped too far, enough of the structural integrity of the core was lost at the level of the fire that one floor simply fell on to the one below it, the rest of the structure above dropping with it. As soon as the first "fall" happened, the sequence was inevitable - the kinetic energy of all the floors above falling far exceeded the design strength of the uncompromised structure below. Because the core below the fire was still strong, it did not twist or bend until crushed by the stuff falling from above which is why everything collapsed straight down. Disclaimer: I was not involved in the design of the towers or the investigation, but I have a degree in Mechanical Engineering, and based on my knowledge and understanding of design and strength of materials it is not shocking (or perhaps even remarkable) that the towers collapsed straight down as they did based on a hot fire burning for some time in and around the core many stories above the ground.

Inside job of course. Carefully placed explosives caused the rapid and uniform collapse of the towers, not the damage from the airliners slamming into them. They were brought down by a very advanced control demolition.