It seems it is still unexplained. It is well explained in the commonly-available texts, but I would like to make a couple of comments. If you are authoring a book or an article please feel free to write to me about relevant citations, etc.
All planets are in motion.
Mercury is a planet. Therefore, Mercury is in motion. Mercury is in motion. Mercury is in motion because it is a planet. Whatever is a planet is in motion, for example Jupiter.