While I don't ascribe to any over-arching conspiracy theory about this, the towers fell VERY neatly. I continue to find that morbidly fascinating.

My personal theory is that at some point during the construction or retro-fitting of the towers, engineers realized that the towers represented a danger to the rest of the area. They were so tall that if they came down sideways or even slightly off center, they could demolish much of lower Manhattan. I believe that to mitigate this scenario, they installed a system that was designed to take the towers down relatively safely via controlled demolition. I imagine that it was pitched as "once we get everyone out, we can take them down safely and mitigate damages and lawsuits." In this case, I think there was an assumption that the design of the towers would buy them the time needed to safely evacuate both the towers and the area that would be affected in the fall. I imagine that also this system was something that was designed to be triggered remotely.

BUT, I do think the system had some sort of automatic trigger with a very high thresh-hold. I assume that the system was originally designed to only automatically initiate this system if building integrity was in imminent danger of failing. I think that unfortunately, that thresh-hold had been designed in such a way that the system may have triggered ERRONEOUSLY.

I think the fact that the towers went down 30 minutes apart might indicate that the system was functioning poorly, or it might be that the collapse from the south tower convinced the system in the north system that it was experiencing some sort of horrible earthquake and it triggered before the next 'shockwave.' I'm not sure what caused it to trigger, if I'm right about this system existing.

I've also considered that it's possible the system was newer than I imagine, determined that the top floors were undergoing catastrophic failure and got the base of the building out of the way as fast as possible.