Regarding "how you did this" I can only guess that you accidentally pulled down the menu and changed it. Or maybe you created this by having a "default action" for "Execute Shell Script" in which this setting was changed.
Regarding "where the global default would be changed", there is no explanation for that in the page cited above, but as I said, if you created this action from your favourites, you can effectively change your "default" because your favourite can be set to any option that you want. So tell me, is the Execute Shell Script action in your Favorites?