Jupiter is not retrograde, but there are retrograde moons among its moons. I guess what you want to ask is why Jupiter sometimes seems to run in the opposite direction to its normal direction in the sky. From the earth's point of view, every planet has this situation. This is because the earth and both of them are in revolution, and this happens when the speed and position of revolution are superimposed. For example, when you take a bus, you look at other cars. Although both cars are driving in the same direction, if you are faster than the other car, you will feel that this car is going backwards. It is easy to understand by drawing a simulation map of the solar system.