Either:
The University failed to do a headcount and hand out only enough tickets for the available seating, such as two per graduate; or
They did issue only enought tickets, but failed to check tickets at the door; so that...
The ones who arrived early in defiance of cultural stereotypes just marched on in with excessive numbers of guests and took all the seats they wanted regardless of not having a ticket.
The latecomers reverted to type instead of asking school officials for a remediatory announcement, such as ”any guests more than two per graduate, please report to the gym, where the graduation will be telecast for you,” or, ”please step into the lobby and sign up to receive a DVD of the graduation by mail...”
But that would have required planning ahead.