Auto Import probably kicked in, and was looking up Lyrics (Auto Import settings and Options) or analysing audio for HDCD (Options).
You could have turned off those settings at the time, and Ctrl+Z would have undone the changes.
If the above was the case, now that it is hours later, you could Restore a recent Library Backup to get back to a point before those fields were populated... although actually those fields are written to the file tags by default, so they will still be there. You could delete that info in MC and Update Tags (from library), if you think it is worth it.
Of course the above is a guess, as I don't know what versions you updated from or to. Or how large your Library is, so I don't know if it was just finishing off updating 30,000 files, or just starting on 1,000 files. Or if you accidentally dragged a directory of music to a new location. Or...
You get the idea. It is hard to know without more information.
