Most swans will chase the juvenile swans from the habitat prior to the next nesting season. The parent swans do not want future competition for mating or habitat resources. Your particular swans may know there are limited resources, i. e., food, shelter, etc., and are removing the juveniles so they can find a mate and more suitable habitats. Also, some individual swans are just more territorial than others.

Yes, unfortunately infanticide (killing of young) is normal in nature. Parents animal/birds in the wild know when something is wrong with their offspring and they don't have the luxury of a doctor to help them. In order to keep a predator from following sick/injured young or a disease from spreading to the family/flock, the parents will often put their young out of their misery to protect the species. The Regal Swan