I always thought it was to do with the gathering of the economy class pax, who are generally in the majority at the gate, as per the Wiki definitions below.

Kettling (also known as containment or corralling) is a police tactic for controlling large crowds during demonstrations or protests. It involves the formation of large cordons of police officers who then move to contain a crowd within a limited area.

Equally, people may leave the "kettle" through an exit point to the south or west, away from the main protest. As the crowd grows and police perceive a threat, the cordon becomes a permanent "kettle" strengthened by riot police waiting in nearby vans. Nobody can enter or leave - possibly for hours.