There may be several things occurring. First, are you absolutely sure that you have a female and male swan? Same gendered swans often can divorce as they get older. Even if a pair is opposite gendered, they may still divorce is somethig better comes along or they are unsuccessful in producing or raising cygnets.

Swans can also sense when something is wrong. Could the female have been injured, swallowed something that may have become an obstruction internally, could she be sick? You might want to have her examined by a veterinarian as swans will banish other swans from a habitat if they know that their mate or offspring are ill or injured.

Is there anything that has changed in the habitat, i.e., any ornament, plants etc., added or removed, are there new neighbors, predators (domestic or wild) in the area? Any significant and sudden change to a habitat could cause a dominant swan to try and remove anothe swan from the habitat if food, shelter or other need is deemed in jeopardy.

If the aggression continues, you may not have any other alternative except to find a new home for the male or the female. Re-mating may or may not occur for any swan, but males tend not to seek a new mate and females will. So, you have the option of finding the male a new home, keeping the female and trying to see if you can re-pair her with another male or give the female away. Either way, if the aggression continues, one swan is better than an injured or killed one due to an overly aggressive mate. The Regal Swan