Ever notice how at 9/11 there was a plane crash into 2 giant buildings and the buildings just towered straight down instead of falling over to the sides? I have some doubts of the airplane being the main reason. And maybe the plane that went missing had something of great importance on it and someone wanted to steal it or the pilot was drinking and driving.
The reason is because the entire floors were heated to extreme temperatures. The building was designed to withstand the impact actually but the steel used was temperature sensitive. It softened a huge amount when heated. Once one vertical coloumn went the weight shifted to the rest because the structure overall was quite rigid. This caused a rapid failing of all of the supports. The airplane did not damage the building significantly, heating the metal supports to over 1000Deg F did.
The scale is also important. At those large sizes things tend to collapse directly downward as the structural rigidity and strength is less per unit weight.