Q. Why do planets retrograde in the sky?
A. Retrograde motion occurs from the fact that the planets are revolving around the sun while the sun itself moves around the hub of the earth. This particular path the planets take makes it appear as if several of them make a loop along their journeys across the night sky.
Please note that the planets are moving very slowly around the sun and would not retrograde several times a day as might be implied by the above diagram. The diagram is for illustration purposes only. Several retrogrades a year would be more appropriate, depending on the planet.
The retrograde happens very slowly in the night sky, over a long period of time.