It has to do with the way you create workshop items. Part of the process of making a mod is to "cook" (or pre-process) the content in the mod so that it can be loaded by the game engine. When you cook the content, you bake into it certain code and data that was in the game at the time the mod was created. If the developers of the game (Coffee Stain) go back later and modify parts of the game that were cooked into the mod (when they release updates and bug fixes), the mod will be referencing things that no longer exist and will crash when the game engine tries to load that missing stuff.