What causes spectator aggresion in sport events

Sometimes an unfavorable result can cause violence in sports events, like when fan or viewers watch their favorite team play, they become easily aroused when their team loses a match. When they want only their team to win, they pass some bad remarks on other team fans that both started a fight between the two.
Another one is referee decision. Watchers become frustrated when the referee or the umpire make the very wrong decision.SO, they pass some bad words to the referee that can start a grudge between the players on the field, then starts violence with the teams players.