Earlier quoted context omitted.
The point of HackerNews is to spark interesting conversation as much as anything. Often that involves going off on a tangent. At least that's why I come here, and I'd guess I'm not alone.
I think it's more people who don't read the article, pattern match on a keyword in the title, and go off on a tangent on that keyword. Article comments should be relevant to the article posted, not any unrelated topic that simply mentions the word.
Well yes, I think that's how the tangents get started. If the article itself is sufficiently interesting or the tangent is particularly off-topic, those conversations get quashed pretty quickly. There were only a couple top-level comments about the mixers, it just happened that there weren't many comments about the article itself to counter-balance them.