Has anyone seen a realistic explanation for "Progressive Collapse"

[Augustine]

this is very wrong (although I like the story) because the conservation of momentum is valid during the impacts, between those floors the block got 3.8 meters to fall and during that fall it increases its speed, and that is a consequence of F=m*a.
It is fact no closed system, g is still working.

I can't see why you cannot understand the accelerating collapse. You have most of the pieces. Exactly, the collapsing floors accelerate between the stories; the energy required to collapse each individual story is a small percentage of the available energy, and consequently the drop in the collapsing block's velocity gets less and less and ultimately there is essentially no deceleration as the collapse wave progresses.