Nobody really knows the indicators that are verified to make us end up in one lobby with 3 stacks of preds or masters (I'm talking about pubs). Because the spread of KD in one lobby is huge and it's definitely not a factor that is taken into account.
I think that the main factor here is the place in the last game - it's very stupid, because you often get 2nd or 3rd place several times and you can't win anything, and it moves you to even more difficult lobbies.
I have the impression that a few seasons ago this MM didn't work like that and it was a bit easier. I know they made changes to MM, but they definitely didn't do anything for good.
As for ranked games, it's always been known that preds feed off of kills on diamond and platinum players because there aren't enough players at that rank at the moment and MM wants to start the game as soon as possible.
At least because they show the rank distribution in the lobby, I made a conscious decision that I'll never play ranked games above D4 again.