What's happening right now in the US with all the shootings remind me of an old statistic about suicide: That in the days after a widely publicized suicide, the suicide rate goes up dramatically. Why? Because many people who have been considering suicide for a while get inspired by what they see to carry out their own suicide. Same with mass murders.
Same when the audience gets a chance to ask a question after a presentation: Nobody wants to go first, but once the first there questions have been asked, everybody just piles on.