Any fight in a public space can be seen and judged by other people. There is always the potential for bystanders to intervene in any capacity – they may stop the fight by using physical restraint or attack someone involved when they perceive their behaviour in a negative way. Quite often, bystanders will actually prevent others from intervening so they can see how the fight unfolds.
In this case, a person intervenes to prevent what he saw as an unfair attack, then left too soon as the aggressor returned.