if you think about like a geolagist thinks about it it goes like this-1.the plane crashes into the building, tragic- but you notice that planes are VERY heavy, and when placed in side of a tower, PRESURE builds on the steel structure.2. A fire breaks out due to the crash, HEAT builds up in the steal structure.3. put it simply it HEAT+PRESURE= explosions, like in volcanos, normally the heat inside a volcano is supressed, but when pressure is added, it europes. same basic princibals here.