Why was LOL Master Yi and Yasuo enemies?

As far as I know, LOL Master Yi and Yasuo aren't enemies. In LOL, Master Yi (ID Estar) was a very outstanding professional player. He performed well in the game and was known as "Master Yi". Yasuo (ID: Asac) was another very popular professional player. He and Master Yi were once teammates in the game, but they also had a short-term cooperative relationship. Yasuo and Master Yi had fought in some competitions before, but they had not become mortal enemies.