I leave a lot of games these days. As a matter of fact, I'm currently on a LeaveBuster for leaving several games recently. I can't speak for others but for me it's frustration. In about 80% of games, you can tell which team is going to win within the first 6 to 10 minutes. Usually, it's because someone on one of the team feeds really fuckin hard. Now, that is just the way it is in season 8, fine. But what gets me is when we have hard feeders and no one wants to surrender. Those games are never fun. It's just 30 minutes of watching your team mates die every 30 seconds with absolutely no hope of being able to do anything about it. Sure once in a while you can make a comeback, but that is extremely rare. So, I started leaving games like that. Sometimes, I would just dick around and let the enemy team take every thing so the game would end faster.
Now, as I said, I can't speak for others but I can speculate that most of us know that games are decided very quickly in season 8 and this is why you would see a lot of trolling, leaving and whatnot. I mean the game is a wait for the first 10 minutes to get a read on who is going to win, after that, it really doesn't make that much sense trying after that of you're on the losing team.
In fact, this is what I do now. I play somewhat passively for the 1st 10 minutes, trying not to feed and I just wait to see which team is going to feed. If it's my team feeding, I know it's done and stop trying or just outright leave. If it's the enemy team feeding, great, maybe I can help carry and secure it. I've tried really hard to figure out what can be done when you have hard feeders on your team and after hundreds and hundreds of games, I've concluded that there really isn't anything you can do about hard feeders. I treat each match like a game of chance. I can only slightly improve my chances of winning by not being a feeder but that barely works. But it's the only thing we can do in season 8.
I've stopped thinking about climbing and all that. I play for fun and if a match is not fun, I just leave and let that be that. I don't even give two shits about LeaveBusters or LP losses. To be perfectly honest, if I find another game to like as much as I liked League, I'd stop playing League altogether. Season 8 is the least fun I've ever had playing this.