I know Hacktoberfest is certainly exacerbating the problem, but as an open-source contributor for ~2 years myself, but someone who has been working on side projects using GitHub for like 7 years now I've noticed a significant uptick in bullshit PRs in the past year or so.
I have a hunch it's because a lot of bootcamps / tutorials / etc are encouraging students to just "contribute to open source." People may either not know exactly what that entails, or may just be just as lazy in their bootcamp as they were in school.
I've gotten PRs on my school projects from 5 years ago fixing typos in comments. I'm not sure how people even find those repos, and I'm even more confused why they'd feel the need to "contribute" unless someone is telling them to without any additional context.