this occurs is initiated by the heating effect of the crashed area......
when the temperature of the fire rised to above 900C, the steel bars would suddenly becomes very weak and soft in texture(due to the change of packing efficiency of iron lattice from hexagonal close pack to simple cubic lattice, which is a much weaker packing form), that can give nearly none supportive force on the building's upper floor's weight (normal reaction recieved by the upper part of the building tends to zero)
This results in the collespe of upper part of building
upper part of building falls, it then crash and "fuse" with the lower levels to become more heavy, the deceleration due to each crash between the falling object and the floor is so small that can nearly be ignored, and the more weight the falling object gains, the little the deceleration is caused and it apporaches free fall.
Also, if the weight of both sides of the building are equal, and ignore air movement and air resistance, the moment of 2 sides of the building are equal, and it will fall vertically.
