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Usually players in lower leagues either don't care as much (e.i. trolling) or don't play often enough to be consistent. You also have your handful of players that are just bad at this particular game, but every game has those.
Personally, I watch far tournaments and streams far more than I actually play. Therefore, I know all the theory for builds/counters/comps,ect but I don't play the game enough to have the mechanics to *always* last hit properly or pull-off crazy combos. I do every once in awhile and it's awesome, but I play casually so I also have my share of fail-flashes, botched skill shots, ect.
I haven't played ranked since Season 2 but I'd imagine that most of the problems in the new league are from players who used to be sorted into a higher division now being paired up with the people who truly belong there. Also, this is a team game so communication and knowing how to properly deal with the social aspects can help just as much as having an above average ability (carrying).
All that being said, trolling is still a rampart problem.