Behold, the Power of Thermite!
I kid of course, about the thermite.
Still amazing though, the incredible pace of the downwards explosions...
Actually, looking at it again, I think I understand this a bit better now than before. There is a bit of an optical illusion here, where the downward falling debris cloud is intermingling with huge explosions from the tower, just ahead of where the debris falling from. It LOOKS like the debris cloud and explosions are moving as an explosive wave down the tower, when it is really the debris cloud moving down plus a few big explosions every few stories.