Theory as to why Riot is nerfing Sona.
As we all know, Sona is getting axed on the PBE 
Now, Sona also had files on the PBE for a new skin, which is quite possibly the ultimate skin. I expect that riot wants to nerf Sona quite a bit before they release the skin, because as we all know, Udyr and Ezreal weren't really picked up until their ulti skins came out. Now, we have 12 year old fanboys afk farming the entire game and screaming about the GAWD DEER whenever they get a kill.
Riot probably doesn't want something like that happening again, but hey, instead of nerfing
, they nerf a medium tier support that, mind you, they chose for the ultimate skin.