What's the science here?
The individual bubbles that make up a head of foam are basically pockets of CO2 trapped by proteins and carbohydrates. A touch of oil breaks up the proteins and carbohydrates, popping the bubbles and collapsing the mustache-inducing layer of foam.
Why the nose?
No matter your level of hygiene, certain areas of the body just produce excessive oils. The nose happens to be one of them, and thanks to its easily accessible placement in the middle of your very good-looking face, it's the first place people reach when performing this maneuver.