Salience of the norms
There is a quirky behavior of humans that psychologists refer to as 'salience of the norms,' which is the tendency of people to conform to social patterns, or go along with the crowd. It is a beautifully cold and neutral description of human behavior, though in study it tends to be used in less than flattering examples.
If you you were to walk down the street with the intention of littering, it seems much more comforting to drop your trash where other litter already exists. Others before you have done it, so it must be ok. You take solace in the fact that you weren't the one to step out from the crowd. Booing and hissing terrible theater or an underachieving sports team, an extreme show of disrespect, is much easier to do once others have already started.
This concept is also seen in the broken window theory of crime. Criminal activity seems much more natural in dilapidating neighborhoods. Broken windows, graffiti, and strewn garbage is a symbol and an invitation to those in the sex or drug trades. The broken window theory goes that zero tolerance on the little things will reduce the big things. If people see a clean and orderly neighborhood, they will be less likely to step outside the social norm of non-disruption. Criminals will be less inclined to set up shop.
I found an interesting example of norm salience in this viral video starring a statued replica of Albert, Florida State University's alligator mascot. Ohio State lost in a friendly contest between the two schools and as per the agreement, would allow for the enemy mascot to be placed on their campus. What begins with common civil respect for property ends with Albert literally turned to dust. This display of incremental violence shows how each act of aggression can be easier to perform than the last, leading up to the inevitable conclusion. Thanks to salience of the norms, no one along the chain of events will actually feel responsible for the final result.
I've been noticing the unique school spirit of Ohio State lately. Not only will they quickly do away with enemy incursions onto their campus, they also know how to intimidate you with their marching band.