This is the problem with voting based social media. A demographic can be entirely supplanted by another really quickly. All you need is for the new demographic to upvote themselves, and the pre-existing demographic to not downvote the other demographic. Pre-existing users will start leaving when they see the content they originally came there for being outcompeted by the content the new demographic has introduced, causing a domino effect.

I've seen this happen with multiple subreddits. HN is also at risk for this, but it's a niche corner of the internet, so it would surprise me if there are ever enough non-assimilated individuals coming in at once to truly endanger the community.