Why some people I know first thought that building's falling was fishy:
1.They could see no damage (but they weren't at the scene)
2. The Saudi citizens, some of them bin Laden family members, were gotten out of the country quickly after the Towers were hit. (I don't remember the date but I think it was about 3 or 4 days later.)
This, I think, led people to be extremely suspicious of the government. So they worked backward and came up with a conspiracy theory. It's just as bad, logically speaking I mean, as when Cheney cherry-picked intel to work backward to come up with a conclusion he's already formed.