It can happen in a flash. Sometimes, all of that situational awareness that is preached just isn't what is going to save you. At times, you will be in a reactionary mode. A lot of things happen in this video.
You can see there are plenty of customers in the store and the older gentleman just reacted and was pretty severely punished for it. If you have the time to react by grabbing someone, you have the time to react by stepping off of center, drawing and firing. The stomp on the neck was bad enough. The kick to the head as the jerk left is a perfect example of why you use lethal force to defend yourself. Let's talk about "unarmed" aggressors. In the video we can see a handgun being used, but that doesn't really matter does it? I once heard someone say that until they stopped hearing "blunt force trauma" as a cause of death in homicides, then they would continue to consider an unarmed aggressor as extremely dangerous.
Something else to keep in mind: train to scan on the range when you are practicing. At the end of the video at the top, I want you to watch that set of feet follow the jerk out of the door. I'm guessing that is an accomplice by the timing and not just a customer running away. Move off of center, draw, fire, assess. Keep your threat covered, but keep your eyes open.