Bison populations were artificially inflated in the Midwest after the collapse of Native civilizations due to disease. All the fertile agricultural land was opened up and the people who would have hunted them were nearly wiped out.

"In the early sixteenth century, Hernando de Soto’s expedition through the Southeast saw hordes of people, but apparently not bison, or he would have mentioned it. More than a century later the French explorer La Salle canoed down the Mississippi River. Where de Soto had found prosperous cities, La Salle encountered solitude without any trace of humans, but he saw bison everywhere, grazing in herds on the great prairies that then bordered the Mississippi. When Indians died, these huge creatures vastly extended their range and numbers. According to scientists, the massive, thundering herds were a pathological symptom, something the land had not seen before and is unlikely to see again. " From a review/summary of 1491 by Charles C. Mann

The pattern of horse-riding bison-hunting nomadic natives only emerged in the 17th century. These people were basically the remnants of agricultural societies adjusting to a post-apocalyptic landscape ravaged by disease. The civilizations of their ancestors collapsed due to European pathogens, spread via direct or indirect contact. Pigs released by the Spaniards carried all sorts of diseases that spread through these more urbanized peoples (Ecological Imperialism gets into that at depth).

Horses released or traded by the Spaniards were incorporated into native cultures to take advantage of the newfound resource presented by the bison in the Mississippi drainage and the new opportunities presented by a rideable draft animal. Patterns of agricultural societies becoming nomadic after developing draft animal husbandry has also been seen in the Eurasian steppe, particularly the Pontic region.