I think beyond the way the site is set up that the sheer volume of users on Reddit is a large contributing factor to the decreasing quality of the discussion and the increasing mob mentality and dumbed down discussion.

This increase also made Reddit a target for numerous political and cultural groups to spread propaganda. There's plenty out there on how China and Russia use social media to great effect and it's no stretch to believe that America and other groups were doing so tok, we know white supremacist groups were doing it as well and Reddit made a great target. Likewise marketers came in too, there's a shocking amount of posts that are clearly to advertise products such as Cola but disguised as innocent that users fall for.

The site used to have far better quality of content and discussion , go on way back machine if you don't believe me, but this changed as the user base grew even though the way the site was designed stayed the same.

If anything I'd say the site needed to change its algorithms and design somehow in order to facilitate discussion better with a larger user base but it didn't so you got the fragmented hostility that caters towards the lowest common denominator as you point out.

Beyond that I think any site , including this one , can be susceptible to the same thing if their user base grew to those levels and honestly I'm not sure what the solution is.