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.
The robbers started this with an advantage of initiative, numbers, preparation and mental preparedness. This is why the ambush is the most effective form of attack. If the shop owner were to react at this time, he would lose the majority of the time.
This shop owner feigns submission, waiting until the robbers are no longer focused on him and draws to fire when they are at their most vulnerable. This is a counter-ambush.
When one of these young men starts reaching into a bag, the other does not recognise the danger he is in, and does not react accordingly. Police pay special attention to this kind of behaviour because they may be reaching for a weapon.
A potentially fatal slam. High risk of severe head injury.