My view is that USENET ran the gamut, depending on what group you were reading. Some would feel more like your typical HN thread or a well-behaved, niche Reddit topic, and others could be just as messy as you could find today. Just in text form rather than images and whatnot.

There is an overall style shift over the long term, e.g. USENET was a little more like email lists and less like chat. More like writing letters to one another rather than having an interactive conversation.

But, those social network problems already existed. There were various kinds of trolls, just like today. Some were just permanently in it for a laugh, others seeming more focused on dealing out grief, and some who (rumor had it) would escalate their newsgroup beefs into real life harassment and stalking. I think there was a period where using real identities, e.g. university email IDs and real names, was typical but then eventually it was mostly pseudonyms whether by explicit blind mailers or just the wave of random usernames at random commercial ISPs.

I don't know what drove what, but I'd say many groups died by attrition. People with niche interests and finite patience started finding other venues like web rings and web forums. Eventually it was mostly trolls, floods of binary attachments, and newsgroup necromancers.