French social psychologist, sociologist, anthropologist, inventor and amateur physicist (dang!) Gustove Le Bon made sense in his 1895 book The Crowd: A Study of the Popular Mind --- whether we’re talking crowd mentality related to markets or, well, hmm:
The first is that the individual forming part of a crowd acquires, solely from numerical considerations, a sentiment of invincible power which allows him to yield to instincts which, had he been alone, he would perforce have kept under restraint. He will be the less disposed to check himself from the consideration that, a crowd being anonymous, and in consequence irresponsible, the sentiment of responsibility which always controls individuals disappears entirely.
The second cause, which is contagion, also intervenes to determine the manifestation in crowds of their special characteristics, and at the same time the trend they are to take. Contagion is a phenomenon of which it is easy to establish the presence, but that it is not easy to explain. It must be classed among those phenomena of a hypnotic order, which we shall shortly study. In a crowd every sentiment and act is contagious, and contagious to such a degree that an individual readily sacrifices his personal interest to the collective interest. This is an aptitude very contrary to his nature, and of which a man is scarcely capable, except when he makes part of a crowd.