Why are my songs being labelled 'MSNG'? It's removing them from albums - HELP!

I've been using itunes for years with no (real) problems, but today it's started filling in MSNG on some songs info. Sometimes in Album Artist, sometimes in Composer, comments or genre.

The result is that it pulls these songs out of their albums (so they're no longer listed with the rest of the album) and it's incredibly annoying! I have nearly 17,000 songs that I have carefully ripped/bought, labelled and rated.

I can't find a result on Google. Is it an itunes issue?

I've recently auditioned Audirvana, Fidelia and Amarra - could it be anything to do with those?

I'm worried my entire library is basically corrupt 😟

iTunes 11.4

OSX 10.9.4