I run a pretty large active forum and we have the same issue, we get a ton of bots or well, did, until we made it so you have to register to see the forums or post or anything like that.
I know extremely little about this kinda thing, but apparently it's something called "web crawlers" and automated spam bots that use Google to search topics and find boards? And based on what I found, the more new content in a place, the more they'll find it and target it.
Google says ~ Forums are usually open to the public, attracting search engine crawlers (e.g., Google, Bing) that index content, often appearing as "guests" in the online list. There is a general increase in bot activity across the internet, driven by AI-generated content and automated scripts, which makes their presence more noticeable on smaller forums.