What caused the climate of the Sahara to change beginning about 5300 years ago

The orbit of the Earth

Explanation:

It is widely speculated that orbital mechanics were responsible for the period of lushness that the Sahara desert and Northern Africa lived only 6000 years ago. Due to the changes in the Earth's orbit around the Sun, it is thought that the monsoon effect (season variations in wind patterns that produce a heavy rainy season) was more pronounced across the northern latitudes and resulted in the Sahara being much wetter than it is today.