Collective Moments: Contagion
Art and Story by Jerome Ishikoro (theOtaku.com's The Mask)

According to Gustave LeBon, crowds behave the way they do due to contagion, or the spread of emotions and behavior from one person to another. When one member of a group gets emotionally pumped up, it causes another person to become emotionally pumped up as well. This phenomenon will spread on to the different members of the crowd, like a cold virus spreading around a crowd after somebody sneezed (hence, the name contagion).

Contagion turns people from being rational individuals into irrational members of a crowd.

Main Reference:
"Group Dynamics" (4th ed.) by Donelson R. Forsyth