This police officer is trying to arrest a youth while surrounded by several others. He struggles to gain control while standing, which is extremely difficult to do alone. After being punched and falling, he receives a kick to the head which knocks him out.
Two men argue with two security guards. Another guard grabs one of the men around the neck from behind. As the other man looks back and is distracted, a security guard standing in front of him punches.
The guards flanked and overwhelmed the two men. Human only have eyes in one side of their head, and a split second of distraction is all that is needed to land a punch undefended.
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.
Gaining top position on the ground creates a significant advantage. It is very difficult to escape or retaliate from there, even with “dirty” tactics. In this instance, the person on the bottom cannot escape so attempts to bite, which only escalates the aggression of the other man who then beats him terribly.
Bystanders choose not to intervene, deciding that the beating is justified. If the man had not announced that the other had bitten him, they may have felt and acted differently. The opinion of bystanders is paramount and they need to be communicated with for this reason.