They're doing this so that they can put "I have x amount of accepted pullrequests on github" and fool an employer to hire them. To become shit team members or outsourced to become shit offshore team members*.
I have seen these kinds in many a project. Real world experience. Real talk.
Yes, I have seen a bunch of those on popular repositories over the years. It is an annoyance, but usually so little, even on popular repos (I contributed much to PHP as example of project size) that it's easy to ignore (or even simply merge if it's somewhat useful, the fooling won't lead them far)
The difference with this marketing stunt is that there is way more active encouragement for that and way more of it at once ... multiple a day instead of one every few months
There is a good amount of questions on SO or tutorial blog posts that are basic, really basic stuff. Things that if you had barely any idea of what you were doing you would figure out but you don't.
Oh and now guess what, this flood of PRs raises the bar and annoyances for everybody.