It turns out this is a different video than the one i had in mind but... Thank you for going to the trouble nevertheless.

It'll be interesting to see what else crops up, but the clip i was thinking of showed police taking a group of antifa looking types through building before posting them through a door shouting something along the lines of "let's go!"

There might be another explanation, but It very much looked like a group being actively let in to go cause havoc.