What is precession cycle? What causes it?
Precession is best seen with a spinning top. It's axis of rotation rotates about a vertical axis. In the case of a top it is due to gravity pulling down on the top.
Likewise, Earth's axis of rotation itself rotates around an axis perpendicular to the plane of the ecliptic. This is again caused by gravitational pull. In this case from the Moon, Sun and other planets.