The entire corner of the building was scooped out by the collapse of two of the largest buildings in the country right next door.
it then caught fire.
As the fire burned the structural steel looses it's ability to support the weight of the building above it.
Once the weight of the building was more than the slowly failing supporting structure could support, the support failed and the building fell into the damaged area.
It's exactly how/why the other two towers collapsed.
Oh what happened to building 7 then?
https://911fantasyworld.files.wordpress.com/2011/05/wtc7corner.jpg
The entire corner of the building was scooped out by the collapse of two of the largest buildings in the country right next door.
it then caught fire.
As the fire burned the structural steel looses it's ability to support the weight of the building above it.
Once the weight of the building was more than the slowly failing supporting structure could support, the support failed and the building fell into the damaged area.
It's exactly how/why the other two towers collapsed.
buildings are heavy.