Faced with two risky options ...
Reading this thread ... several people have tried to relate the passenger behaviour to lack of IQ, intelligence, common sense etc. I'm not sure that's relevant as I'm convinced that many rational people are capable of such behaviour under certain circumstances.
Imagine being a passenger on board, during the takeoff roll you are suddenly faced in your own mind with the unexpected but direct and immediate prospect of being incinerated if the takeoff continues with images of a flaming concorde looming in the front of your mind. Taking the highly risky and extremely drastic step of standing up and banging on the cockpit door even at such a critical point might, by comparison, seem like a good idea.
I reckon those passenger knew full well the serious risks involved in standing up during the takeoff but felt they were facing a far more unpalatable option ....