Could anybody explain the Leaverbuster system?

Because right now it makes no sense to me. I've just received a warning for "AFKing" after a game where, in reality, I had trouble connecting for the first 6-ish minutes, but played for the rest of the game, ended up going 3/4/12 as the support with plenty of wards, CC and damage. And we won that game. So what in the heck did I get a warning for? Someone explain how this works please.
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