Riot has said a couple times that players with a little practice on Aurelion Sol are climbing in winrate much faster than they anticipated. They're pretty sure that if they don't give him some numbers nerfs then he will get out of control really fast once more players have more time with him.
I admit that I see where they're coming from. I've played some games that were clearly against first time Aurelion Sol players and it's not too hard to keep them under control, but some other games with Sols who seem to know how to kite really well with their passive and when they ult it seems like they pretty much get a guaranteed double kill as they start slapping you with orbits with not much you can do about it.