Open Mid is a habit korean players developed because they both aren't retards like the rest of the servers are and don't enjoy wasting their time """"""having fun"""""" and also since most of them don't have the desired PC setup they usually go play in cafes called "PC bangs" where you can play most games on a high end computer for a hourly fee or so.
Basically an open mid happens when the majority of players realizes that either the game is mathematically unwinnable (most idiots playing this game, the same that go yasuo adc, bard mid and other creative picks keep playing hoping and trusting in the stupidity of the enemy team to throw the game and give them an undeserved win, this is one of the main causes why in low elo games in non korean servers this is looked down upon) or because they realize that someone is trolling and thus the game won't be possible to win since the troll can singlehandedly lose the game for his team, so they concur that is better to end the game right there and go to the next one, one of the old habits korean players had was to mass honor unskilled, troll or simply stupid players to publicly expose them so that everybody could know what they're about to deal with, and this reduced trolling greatly on the KR server.
Another important aspect of the "Open Mid" is the concept of "Mental", mental is a term that comes from starcraft, most korean players like better to play in an excited/focused state of mind so that they can have faster reactions, when the mental gets ruined they usually underperform so if many players are experiencing this fatigue they usually open because they believe that they won't be able to perform to a standard necessary to defeat the enemy.
You won't see it often unless it's used to attention whore in chat (the notorious P.O.B. "NA open") or if someone is saying it in chat without the general consensus of his own team, especially because unless you're in challenger/masters people are usually too stupid and clueless about the game that they'll never realize the game is already won and all they have to do is just close it, so there's always a chance to comeback. (You might know this as the "my team can't even manage to win a surrend vote" in western servers.)
But yeah, in every elo composed of serious people when the game has gone down the shitter they usually prefer to not waste time and concede to the enemy team and move to the next one.