Conspiracy Theorists often ask why did the South World Trade Tower come down first, despite being hit after the North Tower and thus obviously burning for less time.
The reason is due to how Flight 175 hit the South Tower. Flight 11 hit the North Tower roughly quite central, whereas Flight 175 hit the corner, and lower down on the South Tower. This resulted in more pressure on the upper floors of the South Tower, and thus the collapse came before the North Tower.
For a full, and probably a more easier was to understand this, check out Screw 9/11 Mysteries.