Another cause of the condition is the perception of a non-dangerous event -- such as turbulence or normal sounds that planes make -- as being an actual threat to one's safety.
Air traffic control personnel "would have known immediately" of a crisis when the planes veered away from their normal route, according to Chris Yates, aviation security editor of Jane's Defence Weekly.
As the planes were taken over, one by one, each pilot would almost certainly have expected a "normal" hijacking - where terrorists demand to be flown to a place of their choice, and negotiations begin.