I would guess this is some kind of a standard procedure for detaining people during a protest. And by detain I don't mean code-word for arrest, I mean the legal term for questioning people when police have reasonable suspicion that a crime has been committed. Due to the presence of an unfriendly crowd, they presumably try to avoid escalation by grabbing the person quickly and having the discussion elsewhere. In the previous link someone gave, the police said as much. If this is the case, then other than the relocation they still shouldn't be able to go any farther than any other stop in terms of searching the person or anything.