Towers just don't fall like trees. These towers were designed to support loads from above, and that's all. How in the world would they have enough lateral strength to support the loading necessary to impart rotational momentum on that big a mass?
Once those towers started falling they were going to take the shortest path toward the center of the earth, straight down. This was a very sophisticated attack, and the attackers would have to know this. It's not rocket science!
Actually, it is probably more complex than rocket science.
Assuming the bombers knew the building would fall straight down...or that the attack itself was very sophisticated... is wrong. A sophisticated, knowledgable attacker would have known that the van bomb was inadequate to the task and would also have known where the best placement was... they didn't.
The testimony of some of the conspirators was that they expected to blow out the side supports "and have the building fall into the second tower."