It could be many things.
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A smurf who's tanking his MMR intentionnaly to climb back after.
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A shared account (with 1 good player playing and 1 or more baddies)
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A noob paying to get boosted. A booster could play a couple of game and ofc win a lot (raises MMR) then the account owner takes back the account and since he's just really bad at the game, loses and goes back to wehre he belongs.
This kind of game is pretty annoying. That being said, over enough game, it evens out (meaning the enemy team will get that kind of player too and you'll get a free win). There's isn't much that RIOT can do other than banning the account. Ofc nothing stops the guy from making another account... banning the IP address isn't a very viable solution... most ISP use dynamic IP now so the banned IP could be given to another person at some point... also people can live in the same house and share internet (thus sharing the same IP).
I wouldnt give it much thought. We all get those game yet we manage to climb anyway. Just forget those game and focus on the good one
[vlad-salute]