Conspiracy Theory: Matchmaking Punishes You For Negative Behavior.
Let me just say, I have no concrete evidence of the title (nor have I tried to prove it or claim that it's true). It really feels however that it is the truth. When I'm a happy, relaxed, chill player who overlooks my teammates mistakes and enjoys life, most of my matches feel like they are fairly even (and look evenly matched by the matchmaking).
Sure enough however, if I leave a match (for whatever reason) or say something critical or negative to one of my teammates, the very next few matches are living hell. It's almost as if getting reported causes the way you are match made to change. I've been winning fairly consistently for about a month now (when I started playing ranked again) and things have been smooth sailing until a few matches ago where I became irritated at the Xerath on my team (for being a bone head who can't land a single skill shot).
I've now lost 3 matches in a row (which hasn't happened in months). First matched against a smurf, then another match with multiple Gold enemies, then the last match getting a first timing Top laner, a Bronze mid, Bronze support, and a 31% win rate ADC. I'd like to dismiss this as pure chance, but it's very difficult to given that recently I've been winning 80-90% of matches, without much difficulty, and then I demonstrate a bit of negative emotion and now suddenly I can't win a single match for 3 games straight (even when still performing well).
At any rate, there's no way to actually know, but I am really starting to believe that Riot intentionally starts rigging your matchmaking if you are a negative player. I know Riot claims they don't do "prisoners island" ( https://nexus.leagueoflegends.com/en-us/2017/01/ask-riot-banished-to-prisoners-island/ ) but I'm honestly finding that more and more difficult to believe. I think that's just a bunch of PR given to make themselves look better (similar to how they try to make it look like they don't mistreat their female employees, yet had a large quantity of disgruntled ones just leave even afterward). Personally, I believe Riot is hiding things from us and lying in order to "keep the peace" and avoid losing players.