Hello there, I am an environmental historian, so I can explain the historical trends that have resulted in desertification of northern Africa and the Sahel. This is also part of a much larger climatological process, involving some hefty science. I would suggest cross posting this at r/askscience for some more in-depth explanations of that.

Desertification is the result of a combination of factors, including soil erosion, deforestation, overgrazing, and periodic droughts. The Sahel, the area bellow the Sarah that is turning into dessert, has been experiencing a prolonged drought (lower than normal rainfall) for about forty years. many environmental processes create feed back loops, and this is one of these cases. As the drought continues, less land is suitable for cultivation, leading to more forest removal and overuse of soil, resulting it being exhausted. This results in more land becoming desert and more intense over-cultivation of the land still available. With no alternative food source, there is no opportunity to let the land recover. Even if that was possible, the larger climate trends would probably still result in desertification, even if it was much slower. Just about any environmental science textbook will provide you with this information. For a reference, I was using Raven et all, Environment.