Having a toxic sub-community leaks into other communities. I've noticed that myself, the /lit/ subboard no literate on 4chan has been taken over by /pol/ users.
It has been shown that when Reddit banned two toxic subreddits, the users of those subreddits either left completely or drastically reduced their hate speech in other subreddits:
>Within the frame RQ1 and RQ2 provide, we find that the ban worked for Reddit. Many more accounts than expected discontinued their use of the site; and, among those that stayed active, there was a drastic decrease (of at least 80%) in their hate speech use
See http://comp.social.gatech.edu/papers/cscw18-chand-hate.pdf
In other words, even if you filter your own subreddits to the ones you 'like', chances are that the toxicity leaks into your favorites.
(This is the same paper quoted in the OP's text)