This is OT... The beginning motion brought the top section and the bottom section together as top descended. It dropped because of insufficient axial support. This was a combination of weakened columns (Mick doesn't/can't model) and mis aligned columns (Mick can't /doesn't model) . However I have suggested in the past... and the cartoon diagram posted at IF shows that as these 2 processes were the destruction of the floors in part was taken place to some extent in both the top and the bottom. The tops floors were locally failing and the bottoms floors at its top were locally being destroyed from the falling bits above. The falling of the upper sections was at least partially related to and involved with the warping ergpo the mis-alignment and missing of columns ends (loss of aggregate strength) and absence of resistance when it dropped. This likely led to some extreme concentrated loads where column ends would meet parts of the structure.. dropping top meeting static bottom... static bottom meeting dropping top and punch through anything in their path.. This might lead to sheared beams or further breaking of the slabs into chunks. It's hard to conceptualize and to know what and how these mechanics were functioning. You can't have missing columns unless the top is moved laterally.