Some plants send out roots above the soil line, although usually it's an indication of something. If bent over, the change in orientation triggers a change in plant auxins that induces roots, since the plant's natural response to bent over stems is to re-establish a new set of roots to stabilize. Roots also have a different "bark" or outer skin, which makes that area resistant to mold and allows transmission of nutrients.
A plant that is rootbound will also force out roots - I've had Wandering Jews and other houseplants do this when the pots their in are too packed.
n_d_shadows has a thread going where they are intentionally trying to force roots out of a living rooted plant (called air layering). You might want to surf over to that thread and take a peek. I'm sure people watching that thread would be interested to hear about your spontaneous root situation
