They just happened to be underage Justin Bieber followers screaming, shouting, and gathering in the street, frustrating both city cops and Barclays Center security personnel trying to get them to disperse.
I wasn't there, but Peter Krashes of the Dean Street Block Association filed a report, with video on Atlantic Yards Watch. At one point, he got into a colloquy with a guy filming a documentary about Beliebers (Bieber fans), who expressed dismay at the situation, agreeing that he'd never seen the impact of the boisterous fans near an arena so tightly situated in a residential neighborhood.
In other words, there's a very small margin for error.
As Krashes wrote:
Hundreds of fans gathered near the loading dock and on the public sidewalks on Dean Street near Barclays Center chased a tour bus they believed had Justin Bieber in it down residential Dean Street. The bus eventually came to a stop near Dean Playground. From my vantage point I could not tell if the bus stopped because fans got in front of it or because the bus stopped to let fans catch up to it.The videos focus on the north side of Dean Street outside the arena, opposite residences, though the first video extends down Dean Street east of Sixth Avenue.
The fans were very excitable. They ran without regard to traffic. They screamed without regard to residents, many of whom were presumably trying to sleep nearby.
Chasing the bus
The bus emerges from the loading dock elevator. The police try unsuccessfully to keep the sidewalk free. The fans excitedly chase the bus down Dean Street without regard to traffic or nearby residences. It stops in front of Dean Playground. The fans see someone named Alfredo in the bus. Alfredo may have been the fellow photographing the fans and waving at them from inside the bus. Deputy Inspector Ameri from the 78th Precinct is on a bullhorn telling the fans to clear the street. A photographer who identifies himself as associated with Barclays Center photographs the interaction of the fans with the bus.
Hanging out on Dean east of Sixth
Fans disperse once the bus leaves. They occasionally continue to scream.
Gathering on Dean Street outside arena, part 1
The fans continue to be excited. They continue to gather near the loading dock and bicycle parking area near residences. They scream when a black SUV appears and when another or the same tour bus arrives. They believe they see someone named Kenny. [Likely Kenny Hamilton, Bieber's bodyguard.] One fan says "Oh my God, I am going to kill myself."
Gathering on Dean Street outside arena, part 2
The fans refuse to leave and remain boisterous even though Barclays Center security implores them to leave. The same photographer who photographed the chasing of the bus continues to photograph the action.
Gathering on Dean Street outside arena, part 3
The fans continue to refuse to leave and don't show much respect for Barclays Center security. A documentary filmaker making a film on fandom says the situation is "not good."
Gathering on Dean Street outside arena, part 4
Fans check out the tour buses and cars stored in the "pad" by lifting the Artbridge fence screening. Some scream although there is nothing to see. Other fans copy them to peek under the Artbridge banners as well. Someone screams "Over there, Oh my God." The sidewalk remains blocked.