The desire for upvotes is largely what makes HN and Reddit approach groupthink so quickly. Folks with an opinion that they know will be popular will hyperbolize their opinion knowing that it will appeal to the dominant opinion of the community and get upvotes. People who feel really strongly about an issue then go into a thread and upvote everything they like and downvote the things they don't. They are often also the people with the most time on their hands. That's why heavily moderated subs (like AskHistorians and AskScience) that rely on a more formal moderation structure with credentialed moderators result in much higher quality discourse.
When you know your opinion is shared by the majority of the site, you'll be less careful about what you write and less charitable to folks with different viewpoints. You're more likely to sneer at the "opposition" and performatively signal about your superiority. That's how upvote social sites, like Reddit, end up getting so many highly upvoted "hivemind-like" uncharitable takes. Likewise, if you know your opinion is not shared by the community. You will hedge your writing, emphasize how it's simply your opinion, and water it down with caveats and complications that while true are a lot less emotionally evocative as the comments written by the majority. Eventually folks with minority opinions may just churn as the stepping around eggshells becomes exhausting. This creates further pressure to conform to the majority.
Some folks with majority opinions may not feel that strongly about their opinion but will post strongly anyway, knowing that they'll get upvotes. I've done this when I was younger as the dopamine boost makes you feel like a community hero, like you're "fighting the good fight." I've actually said some incorrect things due to this in the past but have nonetheless been highly upvoted.
HN has some safeguards here, like not being able to downvote someone who replied to you, hiding downvotes behind a karma threshold, and judicious moderation separate from upvotes/downvotes. But it's hard to change the fundamental nature of upvote based sites and the clique dynamics that form as a result.