Yes, we're back to the "Path of Least Resistance."
Things tend to follow such a path in one of two situations: (1) If it's being actively directed by intelligence, like a mouse running a maze; or (2) if the object is under the influence of a conservative field. The former case is certainly not relevant, but the latter is partly relevant here.
A conservative field is one in which there is a valid potential function. One example is gravity. At any point relative to the Earth, you can compute how much gravitational potential energy a given object has. That number is unique. All you need to know is its position. You don't need to know how fast it's moving or how it got there.
You can think of the potential function graphically. In the space business, we sometimes talk about "gravity wells," because that's how the gravitational potential energy looks -- around a massive object, it's like a sink, with typically circular contours that go around the object. The closer to the object you get, the lower your potential energy.
The potential energy also gives rise to the force. Force is the gradient of the potential, i.e. the "slope." This slope also has direction, however. Think of a hill, and where you are on that hill dictates what direction you'll roll down, and how fast.
Now, in the case of the World Trade Center, that force gradient is one direction only: Straight down. That is the "path of least resistance." There should be no surprise at all that the structures fell, to gross order, straight down.
However, there are other forces in the problem, i.e. contact forces with surviving structure. These, unfortunately, are not conservative forces. Energy and momentum are always conserved, of course, but these forces do not have a single-valued potential function. The force you experience depends on how fast you're going and how you got there. If you're falling very slowly, and the structure relaxes but stays intact, you feel one force. If you fall very quickly, then the structure will snap and you feel a different force. So here, there is no well-defined "path of least resistance."
In order for anything to fall sideways, the structure has to generate a large lateral force. This can happen through, for instance, the mechanism of an inclined plane. But for an inclined plane to deliver this force, the plane has to not collapse. Its force is a response of the contact force coming from above. As we've calculated, the lower structure can't generate such a large force -- even if it's intact, rather than having floors and bracing smashed away.
So there's no reason at all to expect it to go sideways. Straight down is the only way it can fall.
