While Humans.txt has a very well set up and informative website explaining its purposes, highlighting its uses and even showcasing some of them, I can not find any similar details on AUTHORS.md.
I spotted a tweet pointing to a GitHub repo, "Hulk" and specifically one feature of it - generating an AUTHORS.md. This was the source of my curiosity, so I asked the author about it. Not even he was sure about its exact purpose vs. that of humans.txt.
A search on Google yielded only one developer related result, a GitHub repo named ZakharovviHumansTxtBundle. All the other results were related to medical issues. Similarly, searching for "authors.md" on both StackOverflow and StackExchange yielded no results. Not one. Conversely, the query "humans.txt" yields many.
Origin... no idea.
The purpose of an AUTHORS file is, typically, to identify individuals who have made substantial contributions to a project. This seems to be done in a variety of ways. Such files may identify project founder(s), identify a current "development team" (individuals currently engaged in the project), identify those have made "significant" contributions to the project in the past, etc.
A second purpose, for some, is to, through acknowledgement of contributions, motivate others to consider contributing to the project (e.g., see the "encouragement" at opensource.guide in this regard).
Explicit assertion of authorship typically confers legal rights such as copyright. Google has clearly thought about this a bit (e.g., see Google's open source contributions guidelines and additional elaboration on the guidelines).
With the widespread use of version control systems, the identity of the individuals responsible for various components of the source code in most projects is pretty clearly established at a pretty granular level via the VCS metadata. Thus, there is now likely less perceived need to add author name(s) to the headers of individual source files. This might tie in with the second purpose noted above: e.g., (a) perception of "sole ownership" might discourage collaboration and (b) an AUTHORS file establishes a way to clearly give credit if putting authorship information in header files is discouraged for a given project.