Whereas 'mad as a box of frogs' clearly isn't that mad as a box is rigid and therefore easy to carry, a BAG of frogs would be quite insane to transport because of all that nutty hopping and ribbiting, it would slip out of your hands in quite an nutty manner.
Person a: See that Glyn?
Person b: yeah, I know him, he's mad as a bag of frogs.
Person a + everyone else: yeah, I agree. Quite barking mad.