The planet is travelling in an epicycle, therefore on it's orbit around the earth it looks like it is travelling backwards, because the epicycle rotates faster than it orbits earth.
http://www.lasalle.edu/~smithsc/Astronomy/retrograd.html
good source.
The apparent westward movement of a planet is known as retrograde motion. This phenomenon occurs when a superior planet passes Earth in its orbit, making it appear to move backward in the sky relative to the stars.
This motion is called retrograde motion. It occurs when a planet's motion in its orbit appears to reverse due to differences in orbital speeds and distances from Earth.
Neptune has a moon named Triton that exhibits retrograde motion. Retrograde motion is when a moon orbits its planet in the opposite direction of the planet's rotation.
A planet's backward motion in the sky is called retrograde motion. This occurs when a planet appears to move in the opposite direction in the night sky compared to its usual eastward movement.
venus
The "apparent" reverse or backwards motion of a planet as observed from Earth. The innermost planets appear to have a retrograde motion when viewed from Earth.
venus
Retrograde motion is an optical illusion where a planet appears to move backwards in its orbit when viewed from Earth. This occurs due to the difference in orbital speeds between Earth and the observed planet. Modern understanding attributes retrograde motion to the relative motion and positions of the Earth and the observed planet as they orbit the Sun.
The planets don't have to "do anything" to "go into" retrograde motion. The retrograde motion, the "moving backward" that planets do as we watch them cross the night sky over the weeks, is due to the nature of the orbit of a planet and to our view of that orbit from earth.
Retrograde motion.
The apparent backward motion of planets is called "retrograde motion." This phenomenon occurs when Earth passes by another planet in its orbit, making the outer planet appear to move in the opposite direction against the background of stars. Retrograde motion is an optical illusion resulting from the relative positions and movements of the planets in their orbits around the Sun.
A planet isn't retrograde - retrograde means the direction a planet moves in relation to how we see it here on Earth or other perspectives. Planets sometimes appear to move backward due to Earth moving as well.