Whenever you lose and there is a premade on your team, they will 95% of the time complain about you or some other random player. This holds true even if they are the ones who did the worst on the team. There was this pre-made who went 0-15, then complained about item build. Or there are the times where premades die then say "where were you" or "what are you doing" or "do something useful"
The problem with premades is that they can behave toxic to the other player but they don't get reported. Let's assume 4 premade vs one normal guy, how can you report all 4?