Astronomically speaking, retrogrades happen as Earth is passing or being passed by other planets. Because their orbits are not perfect circles, but in fact, ellipses around the sun there is an apparent sense that some planets move backwards from the position in orbit from Earth. Like the apparent movement of the sun through the zodiac, a planet seemingly moving backward is an illusion, as planets always revolve around the sun in the same direction.
The appearance of retrograde motion is also affected occurs by the relative difference in speed of each planet's orbits. It's something like when you're sitting at a stop light and the car next to you takes off unexpectedly—you may find yourself stomping down on the brakes because of a temporary feeling of moving backward. Similarly, it's when planets are at their closest approach to Earth that apparent retrograde motion occurs. In short, retrograde motion is an illusory side-effect of planetary proximity.