A great many people are susceptible to being led, influenced or persuaded when they either want to be part of a group or there is something important to be gained. The scenario here is one that employs suggestion, peer pressure and persuasion.
Mostly there is an initial choice made for this kind of mass behaviour I think, it is a shared experience which enables a sense of belonging and commonality. Very sadly it can be abused to the extreme as in the awful Jonestown mass suicide with people being led by a charismatic and lunatic leader they want to believe in.
It is quite amusing to watch how very carefully some people ‘fall’ while others fling themselves into the plastic chairs willy-nilly! Not so amusing to see grown up people doing this en masse in this day and age. In the UK there is a chap called Derren Brown who practices various kinds of ‘mind control’ but is transparent about his methods. He chooses people who are suggestible, empathetic and affable as his subjects openly explaining that strong-minded individuals are less likely to follow his lead or do as he suggests.
To a large extent I think the behaviour is innate in most people and might be a survival mechanism. Crowd behaviour, mass hysteria and incidences of sympathetic illness in large numbers of people are well documented. It’s a fascinating field.