In addition to the fire the planes caused you also have to take into account the physical damage the two planes did to the buildings.  I am no expert on constructual engineering but a crash of that magnitude must have done something serious to the bearing structures of the two buildings.  

Both planes most certainly destroyed much of the supporting constructions at the impact area.  It was just a matter of time before the stories above the impact area would collapse.  Now, the total and symmetric collapse of both buildings is harder to explain.  It might perhaps have something to do with the increased acceleration of the collapse, still I am sure an engineer might come up with a plausible explanation to this.    

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