If two people are fighting, you first separate them to stop the violence. After they are separated it's easy to see who is still being aggressive. If they are groups, do the same thing if you can. Obviously can be more difficult with groups, but it's the best option.

But this was easy. One group was blocking the other as it matched to a scheduled time and place. And the same group was the only one that defied the officers' orders.

I think it's just that violence has not been typical of political groups in the last few decades, until now. So you're seeing it now when you didn't see it before.