Collective Behavior

The term Collective behavior refers to social processes and events which do not reflect existing social structure (laws, conventions, and institutions), but which emerge in a "spontaneous" way.
The reason why study collective behavior is understanding how people behave in riots, what sets them off, and how they can be rapidly concluded is also knowledge that can result from the study of collective behavior. And understanding how to organize a social movement to pursue social change is one of the areas studied by sociologists.
A crowd is a gathering of people who share a purpose or intent and influence one another. Crowds are a common occurrence in modern life. Most sporting events, concerts, and other performances result in the gathering of crowds.
Theories of crowd behavior
1.contagion theory
Originally proposed by Gustave LeBon (1896), contagion theory proposes that crowds exert a hypnotic influence on their members. The hypnotic influence, combined with the anonymity of belonging to a large group of people, results in irrational, emotionally charged behavior. Or, as the name implies, the frenzy of the crowd is somehowcontagious, like a disease, and the contagion feeds upon itself, growing with time. 

2.convergence theory
Convergence theory argues that the behavior of a crowd is not an emergent property of the crowd but is a result of like-minded individuals coming together.

In addition to this, many various theories exist.

I learn about collective behavior. It is show when many people are crowded, what they doing.
Most of people are follow the opinion that a loud voice or others are following.But I think this is not a simple problem. With this, people should excite the disarray of society like terror or war.