Pretty simple. If you have anything but "normal" behavior score, you are shadow pooled and placed with toxic game ruiners.
Behavior score decreases when you get reported or abandon games. There is some speculation that Valve uses machine learning to monitor users and lower their behavior scores if they are caught doing things which constitute as game ruining, i.e. spamming chat, destroying items, shadow amulet AFK, etc.
Behavior score increases if you get commended and go extended periods of time without excessive reports or abandons.
Can take advantage of this by queuing co-op bot matches, ending the game fast, then swapping commends with all party members.
Repeat over and over with different players until behavior score is normal.
I'll pay 10 fg per player per game. Games can be finished in under 10 minutes, and it benefits you as well by improving your own behavior score.