Preparing on Shabbat for after Shabbat is not one of the 39 melachot (Biblical categories of "work").
I gather that many prohibitions are gleaned from Yeshayahu (Isaiah) 58:13. However, the only thing I could see in this verse is the concept of how to honor Shabbat with one of the ways being the phrase Asot Cheftzecha. There does not appear to be a specific direct prohibition implied, here. Was this prohibition placed as a syag - a "fence" to prevent some other melacha from happening?