Was this a domestic flight?

If so the lack of checking photo ID could have lead to this, example passenger A drops boarding pass in terminal passenger B picks it up, simply boards the plane before the real passenger notices hes lost his boarding pass. Passenger A turns up to the gate, late flights already pushed back as staff though he was on board.

Another option fake passenger, finds the name of some a person traveling that day, goes to the self check-in gets boarding passes for that flight. Then real passenger turns up at the gate, once they have everyone onboard.