Duck & goose mating behavior can be rather rough and violent. Often a onlooker will join in as is happening here. At least this time it was taking place on land. On rare occasions when mating is in the water, several males pile on and the female may drown. This female was being receptive.
Domestic Greylag Goose (Anser anser domesticus)
Chinese Goose (Anser cygnoides domesticus)