Currently guess case standardises various abbreviated forms of also known as to a.k.a. but I don't think the guidance mentions this. Either guess case should stop changing this since various forms are perfectly acceptable or it should be mentioned in the English language guidelines
https://musicbrainz.org/edit/30300230
http://www.oxforddictionaries.com/definition/english/aka
https://en.wikipedia.org/wiki/Aka
https://wiki.musicbrainz.org/Talk:Style/Language/English