I would add to that idea something that would be a totally expected consequence of the presence of stairwells and ventilation equipment. As to the stairwells, pressurized air from a currently-collapsing floor would naturally enter a stairwell at its interface with that floor, and this process would continue as successive floors collapsed. Compressed air pushed into the stairwell would naturally make its way to lower floors, since any substantial pressure increase in the stairway would simply blow open the stairwell doors on lower floors (stairwell doors normally do not latch, and they open only outward, away from the stairwell). The same would apply to ventilation structures, but without having one-way doors to push open. You would expect that if the effect of this sort of thing became visible at all, it would be more apparent near the end of the collapse, because there'd be less reserve volume for the air to be driven into within the floors below near the end of the event than near the beginning. To me, the bottom line here is that there's nothing about a few squibs popping out from weak points on lower floors that strikes me as being unexpected, from a physics standpoint.There isn't much to see at 1min 29sec but later the video shows that there are jets of dust being ejected some floors below the destruction wave. But there are never more than one or two and, given that the tower is already collapsing when they occur, I think it's safe to assume they're a consequence of some vagary of the building's construction - a weak ceiling collapsing on that floor as the building shook, maybe. Or perhaps a heavy object falling through from one floor to another.