League Of Legends is the most played online moba game on the PC and when you are this successful, you tend to attract attention of those who have too much time on their hands.
League Of Legends has recently come under attack from a group of players who are using Distributed Denial of Service (DDS) to make the games horribly laggy for other players. Note that the game has not been hacked and any personal information you have provided to Riot games is safe.
Also worth noting is that the people involved in this activity are active players of the game according to Riot and Riot is actively trying to identify them.
You can read the forum post detailing this casehere.