Edited for clarity and content. For the moment I intend to repeatedly edit as more information comes in or I self-edit for clarity.
As with a lot of ideas that stray from logic and facts, not all the “forced losses” people think the same thing. They are all conspiracy theories in the sense that they take overwhelming natural randomness and attempt to make sense out of it by simplifying it to the point where it only makes sense if there is an entity that is corrupt or incompetent.
Version 1a:
Some think that they give you a MMR disadvantage when your winrate strays too far from 50%. To the best of my abilities to comprehend, they truly think that the MM “knows” how good they are and keeps trying to put them in even matchups. However, when they win “too much” (or too little) the MM starts creating uneven matchups through MMR imbalances in an effort to knock their win rate back down to 50%. These people generally do think that forced wins are a thing as it happens when you lose too much as well.
Version 1b:
Same as 1a except in this case it’s not an MMR disadvantage but rather a composition disadvantage, i.e. you’re given 5 Mercy mains on your team instead of a bunch of low MMR players.
Version 2:
These people don’t think that the MM is omniscient like in Version 1 but rather have a overly complex view of what the MM does and think it pairs people on streaky wins with people on streaky losses to get the system to even out. If you’re on that streaky win then being paired up with a known loser is what they call a “forced loss”. There are no “forced wins” required in this paradigm. Essential to this version is the Gambler’s fallacy (they literally think they’re on a “hot streak”) and failure to understand that a series of wins will garner tougher opponents.
Version 3:
A more complex idea of Version 2 where they believe that the MM uses an enhanced but short term history of performance, beyond the explained MMR system, to estimate where your SR will be and place you with other “losers” if it “believes” you are overranked. Here the system isn’t omniscient as in Version 1 but neither is it as “dumb” as Version 2. Simultaneously the “forced win” comes from the other team who, of course, have ALL been playing well and grouped against you to “force a loss”.
Version 4:
Those who repeat this version seem to understand how it works but fail to recognize the implication in that if you go on a win streak you get put up with harder and harder opponents until you can’t handle it and this is what they call “forced loss”. This is typically a misunderstanding of the capabilities of the system to estimate skill and make matches, an overestimation of their own consistency, and a simple misunderstanding of what would be a normal streak in a truly random system.
Version 5:
I’ve heard several people say that “forced loss” refers to being forced by the thrower or leaver to lose…which is fair, but not really the agreed upon meaning of the term. I gently remind them that “forced loss” generally refers to the MM forcing you into a situation where you are highly expected to lose, not a system where the MM would be fine if it weren’t for a bad actor or bad WiFi.
Version HC:
Not a “forced loss” theory technically, but people who advocate this system think that the system somehow “knows” your skill and give you “matching” opponents to ensure you aren’t “guaranteed” to win. That’s not quite forced losses but rather more of a “forced non-win". Different than Version 1 in that rather than being forced into a 50% winrate through mis-matching of MMR proponents believe that a perfectly matched MMR naturally gives you a 50% win ratio which keeps your SR at a particular level regardless of actual skill level.
Version X:
Some people don’t really think about how they know, just that they somehow do because of Clark’s Third Law: Any sufficiently advanced technology is indistinguishable from magic. Often you’ll just say something like “You play well, you get worse teammates” without any more explanation of how or why. It’s kinda hard to determine which Version they would find most matching their beliefs.
2022 Update:
Version R: Apparently, some people actually believe that the game changes it’s physics and settings to cause you to lose. Why it would cause a person to lose isn’t clear. Why it wouldn’t be noticed by a very large group of people is also unclear. Whether this is a troll theory is furthermore unclear.