I actually don't know. Is there a way to debug this if I wanted to find out? My music collection is all purchased music from iTunes. I re-downloaded the album from iTunes and it was able to process it but that doesn't really tell me why mp3tag is having issues.
I ended up running exiftool with a find across my entire collection of music and it turns out iTunes has been randomly sending me a DRM protected file which was causing mp3tag to fail. I'm having to go back to iTunes and re-download the songs that were sent with DRM. None of the music I buy from iTunes should have DRM. This fixes the issue I was seeing.