This man does not have many skills; but speed, distance management, movement and positioning allow him to use his right hand to great effect against men who possess even less skills than he does.
Most people tend to overestimate their fighting ability. This man is off the mark further than most when he is surrounded by 5 youths (including the one operating the camera). He initiates the violence (flicks a cigarette at one and pushes another) but takes no appropriate action – allows himself to be surrounded, does not move, does not position himself correctly or attempt to isolate and incapacitate his opponents. He appears to falsely assume these youths will cower and back off.
Excessive force on behalf of the bouncers, and idiocy on behalf of the drunks who initiated with aggressive posturing and continued to approach after it was obvious they were outmatched.
The first bouncer kicks his downed opponent in the groin after he falls. The second slaps him down again when he gets up and refuses to leave. A friend attempts to intervene and is punched.
Victim stepped into striking range without watching opponent, did not appear to understand the risk and how fast the situation can change. This was potentially fatal due to the hard surface.
Key mistake is that the police officer fails to keep a safe distance or at least adopt a defensive posture and bladed stance while close to the suspect. Incident appears to be mental health related.
Victim standing with hands in pockets within striking range. Aggressor does not telegraph, strikes while talking to distract victim. Striking from this range happens so quickly that it is impossible to react in time for the vast majority of people.