Simple one for those who went to school in Sweden where this came up during history classes.
"Smör-gås" ("Butter goose) is the butter floating up on top of churned milk, that is to say when in the old days, out at the farm, agitating cream to separate butterfat from buttermilk. The butter was smeared onto bread, and gave name to the entire sandwich (which is also an odd name btw). The goose connection is that it floats on the liquid, like geese on water.