User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-3481504-20130210004157/@comment-3481504-20130212220041

I dunno, possibly she's older than him. And also, Castiel naturally wouldn't recognize Leviathan, at least while in vessel. He might've recognized their true form, I don't know. Well, there could be two answers to that question. 1) There evil, twisted vile souls - who wouldn't want to banish them? 2) Maybe it's fate that they get banished, so they'd want tot enforce that.