In the case of Reddit and HN a lot of it is done by businesses either blatantly advertising themselves or building up the karma they need to effectively do so. I recall reading obviously AI generated replies to news articles written by accounts associated with businesses related to the events in the news. This isn't new in the LLM era. Hobby subreddits are well known to be always full of businesses selling hobby gears and items doing self promotion. It's just that now it is a lot more obvious because of the AI text smell.

That, and probably political astroturfing. Before every election my local subreddit sees a surge of crime stories. Go figure.