Why Did the World Trade Center Collapse? Science, Engineering, and Speculation:

 

According to the article the design of the World Trade Center caused the towers to fall the way they fell. The World Trade Center was design and built in the 60s to mid-70s. The design was meant for new skyscrapers of that error to be less costly as well as very light weight.

News reports notice that when the planes initially impact the towers, the WTC was able to with stand the impact due to its design. The towers were able to with stand winds 30 times more of the weight of the aircraft. But once impacted, the towers were being engulfed by flames in the interior of each tower. An estimation of about 90,000 liter gallons of jet fuel were ignited.

When the towers collapsed many members failed in a way which caused the towers to fall the way they did.