I totally see the "trend" you mentioned coming through and it's giving me a better excuse to stay away from the nonsense in chat. League isn't very fun imho, I don't play unless it's with friends and don't know a single person who enjoys queuing by themselves. osu! is a great solo game and it can give people something to do while they wait for other's games to finish and what not. Streamers (like Dyrus as mentioned) are showcasing osu! which probably brings in a lot of LoL players. The most obvious answer is osu! players like us inviting our friends who just so happen to like LoL to play osu.