I would love to hear your other theories!

I don't think it would exactly be "create a fork of this repo", but if a developer invests significant time and effort solving hard problems where the solutions are implemented in the released source, once an LLM model is trained on it, then someone else could quickly and easily have the LLM generate a new program that implements the novel solutions. Whether this is a problem or not may depend on the motivations of the developer, but this potential for IP laundering may very well begin influencing the licenses and methods of distribution that people choose.

(Of course, I suppose at some point AI will be able to analyze and learn from binary executables or obfuscated source...)