I think there have always been Spring dust storms in the Mongolian deserts...the winds come with seasonal climate change, just as we get winds when Spring comes to our home countries.
However, the dust storms have become MUCH worse in recent years, in the wake of some really bone-headed agricultural projects and policies that took place back in the 1950s, 60s, and 70s. Desertification has claimed vast areas of land, and the Mongolian deserts are growing at a staggering rate.
What was once a local phenomenon now covers a lot of North China, and the dusts- easily identified by their high pollutant levels- are being detected as far away as North America.
The Chinese are frantically planting trees, erecting windbreaks, and doing anything else they can think of to try and turn the tide, but so far no real headway has been made. Weather patterns over North China and the Yellow River valley have been cycling into patterns of increasing drought, making the problems even worse...