RiotJag said in a different discussion
Anyway, let's talk about Sona for a bit. I can definitely appreciate the perspective that she isn't really crowding out support selection diversity. That said, we want to stay on top of the successes and shortcomings of all of our champion reworks, and we feel that some of Sona's gameplay patterns are not the most healthy experience for bot lane. Fighting Sona has a lot of strategic depth that you talk about (pick hard engage or burst characters like Leona to fight her), but we want counterplay to extend past champion select and still be relevant tactically (in lane). The MS change is intended to give enemies without extremely strong engagement capabilities a fair shot at fighting Sona, without exaggerating her weaknesses to hard engage that something like a base HP/armor adjustment would do.
Understandably, Sona isn't perceived as particularly strong, given that she isn't being picked every game (especially in competitive play), and does not seem like she is taking over games that she is in. That said, we have a lot of tools and data we use for examining the game, and in addition to some of the play pattern issues I mentioned, we do feel that she is on the strong side for supports for a wide range of ELOs. Keep in mind, this is intended to be a pretty small nerf overall.
From this it is said that she has been strong, with the only way of countering her was picking a specific champion that counters her. It makes sense to give her a small nerf to make her Q poke a little less harmful, this way she can be countered not just by specific champions, but by outplaying her as well. Sona could easily just push Q and take out your health slowly but surely and then with Power chord just chunk you down and force you to back. This makes bot lane less of a nightmare while playing against her. This isn't a huge nerf, this is just bits and pieces to lower her overall scariness.