Devoid of any retribution, thousands of fans at a football game swarmed the field, climbed on the goal posts, yelled, screamed, and took their clothes off. this behavior of normally mild-mannered citizens is known as

I believe the answer is: Deindividuation

Deindividuation in sociological terms refers to a situation when an individual loss a sense awareness of their behavior when they're surrounded by other members of a social group. In this process, the individual would most likely forget all of their principles for a short period of time and immersed him/herself to the principles that held by the majority group.