I'm guessing "dunny" is what USAians call an outhouse. Doors unsecured when empty.
I think you're right on 'dunny' but I didn't think that it meant banging in the wind, but banging because of heavy use, one person goes out and the next person in line goes in. (Either it's a family reunion, or a family that's passing a severe digestive virus back and forth.)
In short, because someone has a severe case of the back-door trots, as we'd say in the UK.