User blog comment:Rappy 4187/Technical Update: May 13, 2015/@comment-4864110-20150515155728

One other thing, some editors have been using visual editor. There has been an issue with it because when they finished editing an article with it, the categories get deleted for reasons unknown. The editors don't meant to remove categories, the visual editor somehow does that for reasons we don't know why. You might want to look into it.