This happened to me with my last cake, I don't do cakes too often, I had the cake covered and nearly finished decorating it I turned it around and there was a  huge bubble it was at least 2 inches across.  I always depended on icing alone to work as a "glue". But I'm wondering, since the cake was refrigerated and crumb coated with buttercream, should I brush the cake with some kind of edible glue before putting on the fondant?  I do that when I use ganache but I'm not...