There's a lot of reasons for this.
A guy flames another guy, suddenly 2 people are keyboard warriors with no awareness of what's going on.
A guy afks for god knows why, suddenly people on your team don't even change their style of play to reflect the situation and you lose.
A guy decides he just wants to lose, for whatever reason, and plays in ways to ensure that defeat.
But the single most annoying of all, the one that actually tilts me personally, is the guy who gives up after the first blood. Whether it's on him or not, whether your team is ahead, even, or behind; within the first 5 minutes of a game someone will certainly say "yup gg" and wait for the surrender vote that they're so confident is coming.
Why? I get in normals that might be an ok way to play. But often times I ask these individuals, "Why are you queuing up for ranked if you're so ready to just throw in the towel without even trying to come back?" They always respond with the same exact answer;
"It's just a game."
I'm well aware that it's a game. I am. But there's two things wrong with that sentiment. First, you've queued up in a publicly rated system, you know the little badges and boarders and your summoners profile that tells everyone about yourself. Shouldn't you at least put the effort in on trying to win even though you don't think you can? Second, if I play any game I'm not playing to lose, why are you forcing me into a position of a loss?
The longer this game has been out the more people are willing to just simply say "Well I didn't immediately snowball so I'm just going to quit now and whine about every one else's behavior on the team" and then they come here and make posts whining about ELO hell and bans they've gotten.
For the love of god league community, please, for the sanity of every competitive person that plays with you, at the very least if you're going to be a spoiled little brat with a magic 8 ball of game predictions, keep it to yourself. At the very least, please, don't type about it in the chat. Don't spend the rest of the game semi-afk farming while critiquing everyone else's play just because you tilted and don't want to be in the game anymore. Quit or something. That would be so much better than the black hole of lame you become.