A lot of busy, smart people have seemingly random side-projects. For example, Von Neumann:
"A professor of Byzantine history at Princeton once said that von Neumann had greater expertise in Byzantine history than he did" [1]
I don't know for sure why, but I think two possibilities are likely: (1) An extremely strong, natural intellectual curiosity and/or (2) Working on other things allows them to bring fresh ideas/insights to their "main" work, and in this sense is also rejuvenating.