Cheating or system fail?

So there is a thing, i find in so many games: 1 second before the Victory or Defeat message pops up, some player disconnects, and rejoins in the lobby instantly, so he/she is not alt+f4ig, or closing the game anyway, but there is a message with "A summoner has disconnected." I dont know what this is, a system error, or that player tries to hide something from the endgame checking, but for me is just suspicious. I always report those with cheat, but if a moderator, can prove that is just a bug, or some system failure, i'll stop doing it.
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