Jomper, no attacks please. We have been polite to you, you can do the same.
It does not take an application of kinetic energy to make something fall. If you are cutting a 2x4 and you cut through it, the unsupported piece FALLS because of gravity.
When the supports that tied the building together gave, it collapsed, in the easiest direction, straight down.
and thats the part your not comprehending, straight down through the debris isn't the easiest direction, its the hardest actually. It would have to move through itself in order to fall. Far easier to fall off to the outside where there is significantly less resistance