This photograph shows the south tower just as it is collapsing.
It is evident that the building is falling over to the left.
The North Tower collapsed directly downwards, on top of itself.
The same mechanism of failure, the combination of impact
and subsequent fire damage, is the likely cause of failure of both towers.
However, it is possible that a storey on only one side of the South Tower
initially collapsed, resulting in the "skewed" failure of the entire tower.
The gigantic impact forces caused by the huge mass of the falling structure
landing on the floors below travelled down the columns like a shockwave
faster than the entire structure fell.
The clouds of debris coming from the tower, several storeys below
the huge falling mass, probably result from the sudden and almost
explosive failure of each floor, caused by the "shockwave".