Part of the toxicity is because a large portion of the playing community have fallen below the tiers that they think that they should be at. Think of it as one of the consequences of the new ranking system overhaul in Season 7. When people take such a self-esteem hit (even by the fairly arbitrary early ratings in Season 7), then the players could be expected to become more defensive, more likely to blame others for their faults, more frequently picking up and pointing out others' errors, and so on.
Sadly, some people seem to be reveling in the others' pains - which is like poking injured animals. Hopefully, this stops soon. And with that, here's also hoping that the pain ends soon. :)