The media isn't being clear about what really lead up to the argument. But this is what I think happened. She was picking up a call-in order, and went to the front of the line. That's usually permitted when picking up call-ins, and you arrive on time. But you have to be careful how you do that, especially when the line is long, and there's only one cashier. Because the person you just stepped in front of, is gonna feel like you butted, especially if he don't know the rules. But there should always be a second person standing near the cashier for pick orders.
It appears, this dude was the person she cut in front of, and he probably spoke on it. Then she was laying out her rights, but was getting louder and louder. Once the dude understood her rights, he was through with the talking, and demand her to just get her food, and leave. The video started with him telling her to just get her food, which she should have did. But she decided to stay there in his face, shouting, and holding up the line, which was probably pissing off other customers waiting in that line, which was pretty long. That's probably why that one witness was laughing, who I understand was his girlfriend.
Being in somebody's space is questionable, but yelling at them at the same time, is asking for it. Since the dude was about to pick up his food too, he wasn't about to walk away from the window. But he was clearly wrong for pounding on that woman like that, however this was clearly avoidable on her behalf, knowing the man was already steamed up. With that said, I don't blame her son for shooting him once to stop the beating. Although that shot in the back was worst than him beating her, he did what he needed to do, and that should have been it, since his mother was ok. But following that man, and executing him was clearly murder. If he thought the man was going to his car to get a weapon, then he should have gave him a warning this time, before killing him. Then her telling him to shoot his girlfriend too, for laughing, really made this seem like murder, or at least manslaughter.
Anyway, as long as that chick wasn't posing no threat, or talking about his mother, he really had no business striking her, especially multiple times like that, which could have killed her. It comes to a point when a brother need to know when to walk away too. But telling somebody to do all that in his situation is easier said than done. Somebody from behind the counter should have at least intervened before it got that heated. These joints have a history of knowing that customers are often left in complicated situations, for example not be told where the line starts, or which direction the line is going (Golden Corral). Then they stand behind the counter, and let the customers handle it, like they're at a dog fight, or call the cops, who will probably make the situation worst.
Although neither her or her son will be charged, don't be surprised if either one of them pop up in the news again.