He got hit from behind - I imagine he was thinking his presence would put them off and, like most bullies, they would back down at the first sight of aggression. Not true with today’s punks - if they hadn’t knocked him out they would have got a few cheapies in then ran - fact that they got him from behind means that they could then pummel him. Some young men are so weak nowadays they don’t believe they might have to take a punch to be in a fight.
I think that's the Hollywood stereotype of bullies, created in order that feel-good movies could have an antagonist. In real life, bullies are mentally-tough, fearless and can take a lot of hits. That's how they got to be bullies. Some of them grow up to become gangsters like Tommy DeVito. Others grow up to become Genghis Khan.