Two Martian Moons: Newsletter Challenge CR4

Mars has two moons, Deimos and Phobos; both orbit in the same direction. Standing near the equator on the Martian surface one night, you watch them both. To your astonishment, Deimos appears to be slowly moving from east to west relative to you, while Phobos is slowly moving from west to east. How can this be? Supporting calculations will score extra points!