I think the mentality of the Chinese players is kinda similar to the Korean ones. They usually play really well until something goes wrong, then it becomes a "gg get me outta this game" and int/troll.
They are probably used to playing against infallible opponents that really punish their enemies even with the slightest advantage so it feels hopeless to them back home and they lose interest and just stop trying so hard.
The general mentality of western players tends to be more forgiving, the "never give up" attitude is much more prevalent here than it is there, it could also be a cultural thing. Theres a lot of pressure in Asia to doing things perfectly, or to be perfect not just in League but in every day life. Especially for students (the majority of people playing this game).
I also think there's a superiority complex where these players from overseas look down on NA players to a point where if you tell them they're messing up they almost always will troll or disrespect you.
There are also a lot of parachute kids that have too much money early on in life and are basically spoiled brats and that definitely affects their attitudes. Coming from a place where status means a lot its a huge deal.
Although I do not know why all of a sudden theres so many of these players suddenly in NA, I feel like this is the reason alot of them are toxic.
(I use them in context because even though I'm ethnically Chinese/TW myself I grew up here and started playing League on the NA server since forever.)
I definitely could have worded and structured this better but I'm at work and I'm in a rush to jot everything down in form that's mildly coherent.
I hope this helps you understand a little bit better why in general these players tend to be more toxic than the ones here.