They didn't really do anything but play the standard support role, healing and buffing players as needed. The only thing I can think of that might have caused this is Genji's reflect.
You can get both an elimination and a defensive assist if you damage an enemy that gets finished off by a teammate you're healing. With that it's possible to exceed 100% kill participation.
I'm not deleting that for posterity, but it's completely wrong, and the real reason is that if someone destroys Torbjorn's turret (possibly Widow's mines, Junrkat's deployables, Mei's wall, and others, but I can't test) while being healed or buffed by Mercy, they won't get an elimination, but Mercy will gain an assist.