Why does desertification happen?

The main reasons are: -

Overgrazing by livestock. The natural vegetation is consumed faster than it can regrow.

Land clearing of natural vegetation. The balance of the ecosystem is severely disrupted and often leads to soil erosion and barren wasteland due to the lack of binding roots in the soil. This increases loose soil movement and seeds have little or no chance of germinating. Larger and more frequent dust storms also result.

Drainage of subterrainian aquifers such as Artesian Basins. Some of these aquifers used to be very close to the surface where some vegetation could utilise the water. Some also once provided natural springs, creeks and other water sources. Many are now either too low, or have dried up completely, or become too saline or polluted to use.

Salt encroachment of surface areas that turn into deserts due to a rise in underground salt water tables, and, overuse, damming and pollution of natural waterways and aquifers. The once mighty Colorado River and the Murray/Darling rivers are examples of desertification of waterways.

The biggest one in popular theory appears to be natural Climatic changes which are being sped up by man-made causes.