Of course I'm not encrypting it on purpose -- the issue is that I'm finding directories that are encrypted periodically, seemingly randomly, by Windows.
I can't unencrypt/decrypt the directory, either. It appears to be owned by Administrator or some user other than my normal user (and running iTunes as Administrator does not fix the problem). I wonder if my company has some kind of security feature that is spilling over into my external hard drive, even though it is supposed to not be tied to my windows user.
The files are not iTunes, either, but general mp3s that I've downloaded myself, so there should be no need to erase and re-download from iTunes, since I downloaded them directly from the artist.
Yes, I do believe it is primarily a Windows issue, but I wanted to post this in case anyone else having this problem finds this page in a search. I spent a lot of time trying to follow the instructions in If iTunes repeatedly prompts you to authorize your computer - Apple Support only to find that the issue was an encrypted directory, which was not a suggestion on that page.