The most important part of this segment is that nothing is written in stone. The Twin Towers were taken down and so many people were lost because of their planning and an understanding of how those buildings would collapse. However, there have to be situations when terrorists plan to attack and are not successful, even though they have prepared. They may not have prepared enough. There may be a leak of information. Or a group may simply see an opportunity that presents itself in an unlikely and unexpected way, and they may take their shot. There is a certain science to their planning and thinking, but not all of the elements involved answer to science: there is the human element; there is the possibility that things will go wrong. Had any detail of Sept. 11th gone awry, the outcome might still have seen some disaster, but there is no guarantee that an outcome like the plane being forced down in Scranton, PA, might not also have taken place before one of the other two planes in New York hit its intended target.
And in terms of rational? I think the terrorists were crazy: like kamikaze pilots in World War II. They tend to be fanatical, and that does not seem rational as much as overly emotional.