Competition and promises are the main 2
Short grasses that grow naturally on steppes provide grazing for cattle, goats, horses, camels, and sheep. Sometimes steppes are overgrazed, which occurs when there are more animals than the land can support. When the short grasses of the steppe are plowed under for agriculture, the soil can erode very quickly.
The Black Death is widely believed to have been the result of plague, caused by infection with the bacterium Yersinia pestis. Modern genetic analyses indicate that the strain of Y. pestis introduced during the Black Death is ancestral to all extant circulating Y. pestis strains known to cause disease in humans. Hence, the origin of modern plague epidemics lies in the medieval period. Other scientific evidence has indicated that the Black Death may have been viral in origin. (britannica.com)
It was stock market "speculation" that contributed the most to the Depression of 1920-21, since the value of many companies became incredibly inflated.
