These Are The Reasons That Games becomes Coinflips.
AutoFill if u have 3 autofilled teammate on ur team and enemy are all on their main role. u lost the coinflip.
DUO Partners. if one team has duo and the other team doesnt have duo then that game is a coinflip. That Duo Decides the game for other 8 people in the game. if good duo they will 2v8 and if bad duo they will run it down as 2 doesnt matter what other 8 players does.
Smurfs. if ur enemy has 2 smurfs and u have none. u lost the coinflip.
Trolls. Since trolls dont get banned or anything.they are always there. if u have troll on ur team then u lost the coinflip. BUT u can use that Report button after the game i dont know what that button does but its better to use it anyway.
MatchMaking. if Average MMR of ur team is silver 1 and average MMR of ur enemy team is gold 3 u lost the coinflip. BUT instead of 21LP u only lose 17LP. still u lost coinflip.
Edited : Of Course All of These Games Are Winnable. And u can 1v9. But that doesnt mean u won the coinflip. u just did the impossible.