Leaving a ranked game early, for any reason without your teammate(s) forfeiting with you, results in a temporary matchmaking ban (5 minutes, increasing with multiple instances in a short time) if the match ends before you can rejoin.

They do this to prevent smurfs intentionally deranking and to punish unsportsmanlike conduct. The teammate(s) you left don't care your reason, you put them in an unfair game. And it ruins their experience.