What's really happening is that these rioters (not the protestors) are watched trying to instigate violence at the protests and when they leave the area, they get arrested. This is because the local government has been absolutely useless at doing their job at stopping the rioting.
Start at 9:30 timestamp:
https://youtu.be/3ptD6koTknw?t=570
People sharing videos of this share either selectively edited videos without context OR they only share the ending part where they are arrested and not the part where they were instigating violence in the peaceful protests.
Also they are not unidentified. They have Police on their chest.
There's also a theory (can't confirm this) that these are undercover operatives being extracted as they have been infiltrated Antifa.