Go look up "transit" and "surveying tools" on the web somewhere.
They had a transit trained on the obvious bulge, and could actually measure (a term unfamiliar to you idiots who deal only with the subjective) the change in the area undergoing buckling.
At some point, the rate of change made it obvious that the building was going to go.
And go study statistics and engineering a bit. In this case, "A low probability of occurance" means that the building's design criteria did not include protecting against 2 113-storey buildings collapsing nearby, showering it with heavy, flaming debris and causing massive damage and wide-spread fires.
Just as a photon torpedo from a Klingon cruiser chasing the JSF is a low-probability occurance for a fighter aircraft--we don't design for it. If it should happen, though...
We would ave to analyse the wreckage (if any), and the most likely hypothesis would have a low probability of ever happening.
The space-shuttle disasters are good examples.