Okay here's some more alternatives:

Someone up on that restaurant at Elevation 101 sneezed real hard at just the right place.
Reason the other one came down - sympathetic reverberations (and a second bloody 767).

Tuning fork effect. Whenever two tall towers are placed next to each other over a common foundation, they vibrate at a fundamental frequency. At some point this results in catastrophic weakness at nodal points. Given the slightest provocation, someon falling over, a dropped pen, a dropped call, a 767 flying into the side of the building, a collapse will ensue.

It's a little discussed fact, but there are quantum effects registerecd on objects when they are looked at. While the quantum effects are not cumulative, the effect of staring at something and drawing a focus on it have reciprocal effects. So it wasn't really the 767 jet airplanes flying into the WTC towers which brought them down, but the act of all of the people looking, staring, photographing, tuning in on, them that broke the camel's back. We ourselves brought down the World Trade Center.

When the bloody great 767's hit the buildings, a lot of people were rendered extremely nervous and before they hit the stairwells, many o them went to the bathroom. The water pressure rose and fell precipitously with thousands of synchronous flushes and many pipes were brought to the bursting point, critically weakening them and putting the added burden on the steel structure, which would otherwise have withstood the crash of the 767s and fire following.

Monster almost invisible ants, the result of too much carbon dioxide in the atmosphere. (Aided by the impact of two heavy fuel laden Boeing 767 jet aircraft).

Higher than normal gravity waves (they were temporary, too). Gravity follows large aircraft as they cross the sky overhead, or, in this case, fly into high rise buildings.