Originally Posted by Rochard

If you read about how the towers were constructed, it's rather interesting. The towers were build around the core which housed the elevators. If I recall correctly, the floors more or less "hung" from this core. The outer shell also helped to support each floor. This design was used to maximize open floor space - No need for supports. When the core was destroyed from the impact of the plane which also damaged the shell, there was nothing to support the floors. Eventually the upper floors fell, and when they fell they fell down. They wouldn't fall sideways and tip over.