It could have been a couple of behaviors. One explanation could have been mating. If the birds are young and this was there first attempt, they may have been clumsily attempting to mate and both were somewhat upset in not knowing what to do. The male may not have known how and where to grab her neck and the female thought he was trying to drown her. The second possibility would be this was a fight and the dominant swan was trying to get the other to submit to its authority as the alpha bird. If this is the case, there could have been a drowning or at least an ensuing fight with the submitting swan banished from the area. Also, this behavior can continue throughout the two birds' lives if this is fighting and not mating. One bird will always try to be the alpha bird unless one is severely hurt, killed or leaves the immediate area. If new birds appear, this hierarchy fight will continue. This is how alpha birds keep rivals from their family and habitat. The Regal Swan