Of course the Atlanta burning scene wasn't natural but my point was, you can see a lot of stuff falling. Some came down on its own and some was brought down with the use of external force. If you've never studied the way things fall .. there's a good source for falling objects, triggered by fire.
As per your question of corner standing/blocking the expulsion of debris while ejection continues down below through the walls ...
Well, we know that the building had continuous columns running through from bottom to top and the corners had their own lattice work to provide flex absorption.
If the top of the building was dropping naturally, without any assistance from explosives ... and it was dropping virtually straight down into the bottom section -opening it up in a telescopic action, (which it would have HAD to if it went straight down and popped floor joists off of the external wall skeleton) ...
The the top section would have pulled the corners out too, because the top section and bottom section were JOINED at the corners! There was no severance on that corner and metal doesn't just snap off like a toothpick.
With all the floors collapsing and the front building wall bellying out because of that -assuming a natural collapse- then there is no reason or excuse for the CORNER to somehow miraculously remain standing all by itself ... especially when the top section corner is coming down and it's ATTACHED to the bottom section corner.
That means the shell of the building is INTACT below the collapse line .... and all ejection coming out of the windows is explosions occurring inside, BEFORE the top section has even fallen that low. In other words, bottom line .... the explosion speed is beating gravity at this point. Explosions are occurring downward FASTER than that top section is falling.