Plane hits, rips through structure taking out a significant , if relatively small amount of structural integrity (10% approx) and debris removes spray on fire insulation of impact floors
liquid fuels ignites combustibles over half a dozen adjacent impact floors
heat causes steel to rise in temp to the point that it loses up to 50% of its ability to resist loads in turn causing creep of columns and sagging of floor trusses.
Fire spreads to more floors and to other areas of already involved floors causing further heat induced chnages in geometry
Structural loads are continually shifting as geometry of structural components change
In areas where steel does manage to begin cooling much of it is compromised by changes in geometry. While other areas heat more strutural members are heated andbend,creep or sag.
Shifting loads begin causing some structural members to completly fail causing loads to shift abruptly, causing further failures causing more load shifting, etc and rapid failure accross one or more adjacent floors.