update
I realize this is very old but the same things happens to me and I can shed some light onto the subject though I still don't why it happens or how to stop it from happening.
"changing the info" is a little vague, I've never had anything already typed change. It's more like it "fills in the blanks"
It also 'seems' like it only re-fills/restores blanks that I deleted.
I'm very strict with how I keep song files on iTunes, I always like the composer and comments to be blank, and the same goes for genre and album artists (with a few exceptions.)
I don't like the bulk of songs to have a genre aside from a few custom ones.
If import a song it already has 'Rock' listed as the genre, I will most likely delete it and leave it blank.
After a period of time when I sort by genre somehow it's listed as Rock again.
*note*
-This is only a relatively small portion of the files, it does not happen to every song genre I deleted
-I also can't be 100% positive that this new genre (which definitely wasn't there before) was the songs original genre, but I'm willing to bet on it.
To my knowledge a blank the was always blank has never been filled (ex: a song that never had a genre has never suddenly been given one)
Again I can't always be sure what originally had a genre and want didn't, UNLESS it was uploaded from an old CD that doesn't have any song info.
And I'm positive the the songs the changed were all downloaded ones and not ones from a CD.
I've never had custom content change
Ex: instead of deleting the original genre "Rock" I changed it to "German Rock." Even when the composer and comments are restored. German Rock was still the same.
Although on rare occasion the album art was deleted. It has never deleted or changed info I added or altered like the correct song title.
Which is why I think it only fills in blanks.
I've had this blank filling happen on two separate computers (both PCS), and since most people I know don't even look at the info or think to look again to see if it reverted if they edited it I wonder if this is common but unnoticed.
My theories (which are total shots in the dark I am not an expert)
-It's something encrypted in certain files, particularly ones from questionable sources, especially since it usually restores comments that were site advertisements or codes (like 00506 08780...)
-Or something the triggers it to draw from a back-upped memory during an update or something like that
After two years of dealing with it, I finally did a search to see if there was any prevention, and this was one of the few topics talking about it at all.
It seems there's nothing that can be done but maintenance
To see what has filled in, First make sure you have the column for genre/comments/composers/album artist visible, then sort alphabetically in each field and delete ones you don't want.
It is a bit of a hassle but it does seem to happen less and less frequently each time.
Also on the left hand corner click on Edit->Preferences->Store
Uncheck the option that automatically uploads Album Art (if it isn't already) this seemed to lower the occurrence for me, at least for the disappearing album art, but maybe it was a coincidence