Despite the victim’s backpedalling and attempt to prevent his opponents getting behind him, one fades to the rear until he is outside the victim’s vision. The victim loses track of this person, which is when the blind side attack occurs.
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.