Answer:
Improper farming practices, for one.
Farming practices are improper when crops are continually grown, harvested and the soil is not given enough time to replace its nutrients.
Excessive ploughing of land may also cause soil erosion.
Animal grazing may also cause the lack of natural vegetation (forest), and, hence, lack of plant roots to bind the soil particles together, and this will cause soil erosion.
A term to remember is accelerated soil erosion, the removal of topsoil, which contains the nutrients that sustain vegetation growth.
I believe these human activities, which are all classified under improper farming practices, cause (human-induced) desertification.