Apparently, the shock wave traveled up the smokestack, causing it to disintegrate near the top before it "collapsed into it's footprint", to use well-known terminology.
By shockwave, I don't mean waves in the air - I mean waves travelling through the smoke stack, itself.IIRC, the WTC 7 "puffs" or "squibs" seen on WTC 7 occurred in a bottom-upwards fashion, at least from one view. (I don't mean that they started from the bottom of the building; they were all near the top.) If I'm recalling correctly, then this would be consistent with an explosion many floor below, traveling up an elevator shaft, e.g., and disintegrating it's conduit near the top of the building.
I suppose this is where maximum wave reinforcement would occur, though the major factor may have simply been that the smoke stack was weakest near the top.
