how was harris about to fall from the boat

On Page 154, Harris got into a scuffle (or so he said) with some swans. They angrily attacked and tried to pull him into the river..... but he survived.

"The swans had tried to drag him and Montmorency out of the boat and drown them; and he had defended himself like a hero for four hours, and had killed the lot, and they had all paddled away to die."