As shown in the video below, narrowed Dean Street outside the arena--remember, there's non-AY construction on the south side, starting at Flatbush--made for something of a logjam as a bus pulled out after making a delivery, while another (for NBA draftees and others, I was told but not confirmed) waited to enter. This was shortly before 6 pm, for a 7 pm event.
I'm told the video did not capture a longer period of slowed traffic before the video started. A fire truck returning to its Dean Street station was delayed, though it wasn't an emergency.
Note that in the video above, a van improperly parked on the south side of Dean across from the loading dock hinders turning by the bus. It's unclear whether that was a vehicle for Barclays or for the police, but it wasn't supposed to be there.
Also, I'm told, there were a 311 complaint filed about a van sitting at a fire hydrant on Carton Avenue for two hours. As we've learned, the arena is a tight fit.