About building 7
When a bat hits a ball it is called impulse, no high rise building is designed to take an impulse like a jumbo jet flying smack dab into it. If you look at the photos of the building being hit by the jet you can actually see the building bend at the impact.
The impact damaged some of the major load bearing members, and the fire did the rest. Jet fuel is not gasoline at the pump. Standard jet fuel has an octane rating of 100, and can burn at up to 1500 degrees Fahrenheit. Steel loses about 50% of its strength at just 550 degrees Fahrenheit.
The combination of the impulse, and the fire caused a load failure and the floors began to pancake.
Building 7, being much smaller and more frail, was more sensitive to the above. It also had no central steel core and not constructed as a high rise would be for such tremendous impact. The shock and the destruction caused the damage. You need not be hit by the cannonball to be blown to smithreens. Close counts in handgrenades.
In your experience, when explosions like this have occurred, have other buildings fallen?
Box-shaped structures are also inherently weaker structures than things like domes. Forces tend to concentrate at load points rather than being distributed through the overall structure, sometimes even balancing themselves, like trying to crush an eggshell with uniform pressure. Weaken the structure at those load points and overall collapse can follow fairly quickly.