I don't have access to HN's or Reddit's data, but I suspect that it only takes a few dozen people to accomplish the "heavy lifting" you had mentioned. This is particularly noticeable on city/state subreddits, where you quickly notice who 'the usual suspects' are because they comment so consistently and show their ideological bias so readily. These folks may or may not be coordinating on a discord, but I ultimately think they have an activist mindset and are driven to try and control the discourse on the community they see as "theirs".

I've seen people in smaller city subreddits try and stand up to these folks, but they end up getting about a dozen downvotes on all their posts. They probably don't have the energy to keep putting up with all the contentiousness and they almost always give up trying to argue and the echo chamber is restored.