Victim was a black guy
These “kids” are always primed to go off. The guy might have accidently bumped into one of them (they usually sprawl their legs out into the aisle), for example, and not apologized in a way they thought was sufficient. Or maybe he even asked if they could move; it looked like he had been trying to get off the train or at any rate was standing near the door.
They’re always angry. They get up in the morning angry, they’re angry all day, and it takes nothing to set them off.