After being on the top for almost a year, consistently being one of the highest winrate supports, Sona got hit hard by Riot.
This entire time not many were aware of the true power of Sona despite her having 50%+ winrate against EVERY SINGLE other support (which is/was kinda broken honestly).
She was the highest winrate champion for several patches, but if not the one with the highest winrate, she was almost always in the top 5. And for some reason nobody complained about her, she was hiding in the shadows while everyone was complaining about Akali and Irelia. No nerfs, no buffs, she was completely invisible to Riot while destroying solo queue.
That was until the time some EUW duo players showed her full potential, then pro players realised she's really strong. She got played in competitive games more and more, and it got to the point where her strength was completely leaked, now has a 80% pick/ban rate in the LEC summer playoffs.
Poor Sona wasn't able to hide from the Riot balance team anymore. She managed to hold on for a very long time without getting nerfed, but with all the international pro attention she's getting, she can't keep her power in secret. Who knows how long her reign would've lasted if it wasn't for pro players?
(For real though, I'm a Sona main and even I admit she needed and deserved some nerfs.)