Basically this has happened to me in a few games and its REALLY annoying (like seriously GAMEBREAKING). I join a beginner level Co op vs AI game, and anytime i hit an enemy champion all enemy minions around me stop attacking my minions and instantly lock on to me. This is really annoying because it often winds up killing me when any other time the kill would have clearly been mine, and just now it cost me a win.
Im not sure as to what causes this bug, or if its anywhere other than Co op Vs AI beginner games, but its really annoying and needs to be fixed. It happens only in the above mentioned gametype, and it happens randomly, so walking through the steps to enter a co op vs AI beginner level game would be redundant.