A swan generally attacks by striking with the bony knobs on the front of its wings. In at least one case of aggression against a child, the swan seemed to deliberately hold its victim underwater to drown him. That may explain the Illinois man's death.
'Killer' swan attacks, pursues a Chicago-area man until he drowns - latimes.com:
Swans "are territorial and can be quite aggressive when threatened. This aggression makes them good at driving off geese; it can also lead to tragedy.
Anthony Hensley was a 37-year-old married father of two who worked for a company that uses dogs and swans to shoo pesky geese from properties in the area. Hensley had taken to a kayak Sunday morning to check on the swans in a Des Plaines-area pond when one of the larger birds turned on him, the Chicago Sun-Times reported.
Cook County sheriff’s investigators believe Hensley either got too close to the swan or the swan’s nesting area."