Why did the Sahara experience a period of rapid desertification?

Context: The size of the Sahara has historically been extremely variable, with its area rapidly fluctuating and at times disappearing depending on global climatic conditions. At the end of the Ice ages, estimated to have been around 10,500 BCE, the Sahara had again become a green fertile valley, and its African populations returned from the interior and coastal highlands in sub-Saharan Africa, with rock art paintings depicting a fertile Sahara and large populations discovered in Tassili n'Ajjer dating back perhaps 10 millennia. However, the warming and drying climate meant that by 5000 BCE, the Sahara region was becoming increasingly dry and hostile. Around 3500 BCE, due to a tilt in the earth's orbit, the Sahara experienced a period of rapid desertification. The population trekked out of the Sahara region towards the Nile Valley below the Second Cataract where they made permanent or semi-permanent settlements. A major climatic recession occurred, lessening the heavy and persistent rains in Central and Eastern Africa. Since this time, dry conditions have prevailed in Eastern Africa and, increasingly during the last 200 years, in Ethiopia.

Answer: Sahara experienced a period of rapid desertification due to a tilt in the earth's orbit.