Why do hardcore football fans behave like rutting stags?

Football fans can be likened to rutting stags, in that much of their hostility and cajoling is ritualistic. The to-and-fro around stadia, and the local streets and pubs on a match day, demonstrates this nicely: fans tend to stick to streets or pubs they know they’re welcome in (or are made to stay in by police for high-risk matches); shouting and throwing objects often happens as two groups walk away from one another; and group members nearly always step up to pull one another back. The idea is that fans understand the rules enough to let off steam, safely. However, sometimes, these ritual boundaries, in all their subtlety, can collapse.