The east penthouse falls into the building after the columns underneath it failed. They failed because of a localized collapse of some floor around them. That happened because thermal expansion of the beams underneath the floors knocked loose a crucial truss.
The fires expanded the long beams.
You can see them on the right here. The bottom of this diagram is south, the right is the east.
The building was built out over the WTC access ramp. Earlier planners didn't expect that, but the designers of WTC 7 thought they could grab that extra space and get more cash out of rental. So you get those long beams. They designed a truss system that would support the weight of the building, but there turned out to be a fatal flaw in the design. NIST found that simply removing a three-floor section of Column 79 (you can see it in the diagram) would cause a complete progressive collapse of the building.
The east beams expanded thermally and knocked loose the truss that connected 79 to the north wall. That kicked off a local collapse around column 79 of Floors 8-14. 79 then buckled, bringing down 80 and 81 with it and causing a larger collapse inside the building of the east structure. Above, the east penthouse fell into the building, starting the obvious external collapse we've grown familiar with seeing here in this group.
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