New accounts and Ranked placement.
If you have never played a ranked game you should never be placed higher than bronze 3. Even if you go 10 W -0 L and 1000 kills a game. You should start in the lowest tier there is. Because if you are that good you can make it out of Bronze. Time and time again I get a Silver player that has 2 W and 20+ L and is in silver feeding up a storm and making the game so bad.
Riot think you should change the way you are ranked with a fresh account it really is BS that people that bad get into silver when I had to climb my way there to just get but back into B1 because of them.