Just speculating here. And building off some half-remembered On The Media pieces. But as (for any number of reasons) the faith in a social platform collapses (and often the safeguards against toxic stuff go down too, a la Twitter), people with a low tolerance for that shit are likeliest to bail and the 'call people the N word on Xbox' folks are likeliest to stay behind. People who could watch what Reddit did and stay behind aren't bad, I'm not saying that, but their tolerance for unethical stuff is way higher than the tolerance of those who decided to find a new home. Self-selection, ya know.

It is driven entirely by the moderators. r/Canada got taken over by alt-right moderators which attracted alt-right redditors.