What is the psychology behind a witch hunt?

[staisil]

A witch-hunt is a search for suspected witches; it is a type of moral panic.

A moral panic is a mass movement based on the perception that some individual or group, frequently a minority group or a subculture, is dangerously deviant and poses a menace to society. These panics are generally fuelled by media coverage of social issues (although semi-spontaneous moral panics do occur), and often include a large element of mass hysteria.