[Classical Civiliastions] Euripides' Bacchae

I don't really need to knwo this, but if anyone knows the answer it would be helpful. At the end of the play, why on earth is Cadmus cursed to turn into a snake? It doesn't seem to make any sense, as Cadmus was for Dionysus all along!
My edition of the play is most unhelpful here - in the notes, it says for an explanation of this prophecy see Professor Watling's (or something like that) edition. Which we don't have.