Toxicity is high because Riot keeps encouraging toxic champs that literally you can get double by punching keyboard. Since when is an early game powerhouse like Xin rushing FH can outburst a Rengar at level 6 and still outscale Shen building full tank +1 damage core will outdamage your entire carry line. Xin is only the tip of the spear. Yi can build tanky, still dish out the same damage he used to dish out when he built full AD against people who built FH/Randuins and other good armor. These don't even cover really op champs like GP and Poppy.
Dynamic queue is also another aspect as well as this rank system in general. Dude back when we were in the ELO system, you lost rank for EVERY loss unless it was the very rare loss prevented. Now people complaining about going to the promos for the 30th time? If we were back in ELO system that means you would have dropped massively. The current rank system gives people the illusion that they are good. My belief is that almost everyone in this game sucks and the only people who are really considered good even are on the challenger level and high diamonds (or i guess it's masters now?). The problem is Riot wants to cater to the entire LoL community but majority of the community is in bronze. While most of their intentions are pure, many of these people simply look at biased POV therefore what they want and their opinions are not credible. I mean c'mon. I remember people were complaining about how overpowered Nidalee spears were and how overpowered Cho's rupture was. How people rage and tilt if you don't ban blitz in low ELO... blitz is never banned in my games. Because no one's fkin scared of getting hooked. Unless they prove they are amazing at grabbing, most blitz are easy to dodge. However, since it is significantly easier to rank now people who normally should NOT have ranked up get the illusion they are getting better.
Next, I love how there are perma bans on bigotry and racism but not on people saying "kill yourself" or other crap things. Ironically people say the n word in game more than ever. Every fkin game I hear the n word. Not that it offends me. I just don't get the priority Riot has.
With that said, i don't even like that word toxic. I think even the most toxic people were innocent and thinking before the game "I'm gonna win this one, there won't be any arguments, w.e." I think every toxic issue can be resolved by hitting the mute button; not enough people use it or take advantage of it. I know I could stand to use it more than get baited into a ragefest circlejerk.