1) Afaik there IS a new player matchmaking rule in place (it used to be something like lvl1-20, no more; anything above goes into the main pool), but it might be ignored if there are simply not enough players to fill the lobby.
2) Casual MMR is very loose and fluctuates greatly. You get good game - you play with much better players next game, you get 1-2 really bad games - you can go back all the way to the bottom in no time.
Still, to be put into "predator" lobbies after just a few games, you'd need to pull at least one or two 3k+ damage 10+ kill wins. If you're actually slaying like that, why do you think you belong in those "noob" lobbies?
And if you got 1 great game by accident, no worries - you get destroyed in a couple of games with 0 damage and you're back in average-town.
P.S. Kills <> skill. Some1 with 10k kills could still be a hardstuck silver player (and there's nothing wrong with that; and I'm also sure that player is glad he's not matched with predator players just because he played a lot - THAT would be unfair). Don't judge your opposition by just how long they have played.
