This happens because the tags are not saved in the files themselves, the information is held in the database only. When you move the files behind MC's back, and then re-import them, MC believes they are brand new files and treats them as such. The now obsolete entries in your library are removed along with all your hard tagging work.
You have some options:
If the old path and the new path are both in the auto import list, then MC can automatically fix the links to the files if the "fix broken links" option is turned on.
If the directory you are moving them to has a fixed 'base path' (always sub-directories of E:\Video\ for example) then add the base directory to your auto import list, and then let MC work things out for you. If you use external hard drives, network drives or flash drives, be sure that MC is set to protect network files.
You could also do this manually, but in order for that to be safe, you must have the 'fix broken links' option turned off. After moving the files externally, MC will then not be able to find them. You would need to select those 'broken' file entries yourself, then use either the "Update library to point to a new location" option in the "Rename, Move & Copy Files" tool, or the "find and replace" tool, working on the [Filename] field. Both of those tools (use the search option in Options to find them) are reasonably intuitive to use, especially so for the "find and replace" tool.
You could use 'drag and drop' from within MC to drop the files on their new location in the My Computer branch of MC's tree and choosing the move option from the menu that shows when you drop. MC will then move the files for you, keeping the database up-to-date at the same time.
You could export all data associated with those files to an mpl file.
Select the files, choose "File > Export playlist" and export all fields to an mpl file.
Open that mpl file in a text editor such as notepad and edit the filename entries so that they match the new files' locations, save your changes, then use "File > Import playlist" to import the mpl file, which in turn will import the files plus their associated data.
-marko.