Alright, lets go over this scenario.
*Rioter pulls up some Janna stats to decide on nerf*
-Lowest death count in the game overall
-Historically high winrate
-Death count of team drastically reduced when shes present on a team
-Melee champions like Renekton have drastically low winrates against her
-Comeback potential against her is drastically reduced
-High amount of wards placed and killed because its so difficult to punish her for warding/removing wards
Riot Ponytail - "Alright, I've decided what we're gonna do. Lets nerf a single ability but compensate her in pretty much every other way that matters hehe xd"