At first, the pull requests did not make sense at all. One of them made minor changes to a README, e.g., changing "this book" to "the book". It was not fixing a typo or incorrect grammar. It was merely choosing a word different from the one I had chosen. In fact, I preferred "this book", so the pull request (PR) was inconsistent with my preference. There was no explanation whatsoever regarding why this change was warranted. Then I looked at the PR author's profile and found that the same person had submitted several such trivial PRs to other projects too, all of them changing "this" to "the" at some places in README files.
It all began to make sense when I looked at the calendar. It was Oct 01. This looked like PR spam due to Hacktoberfest. For now, I just labelled the PRs as "invalid" (as suggested by https://hacktoberfest.digitalocean.com/faq/), closed the PRs, and moved on.
Like all good things created with good intentions on the Internet, spam is hurting this event and bringing bad reputation to it. The possibility of large scale, endless spam should be worked into the design of any new Internet-based event or solution.