It must have been colassally bad.
My theory is that at least in some cases the people who jumped saw the floor behind them collapsing into an inferno.
Or perhaps they could hear the tower's structural elements failing and feel the tower shudder.
I don't think something as big as those towers will fail all at once. Even though we couldn't see it in video images taken from the ground, the death of the buildings was foreshadowed by many frightening indications that would have been unmistakable to anyone with even a modicum of technical knowledge.
There were many, many such people in the tower that day.
Damn those b*stards. Damn them.
(steely)