Context: In the early 1980s, a small but vocal segment of anthropologists and archaeologists attempted to demonstrate that contemporary groups usually identified as hunter-gatherers do not, in most cases, have a continuous history of hunting and gathering, and that in many cases their ancestors were agriculturalists and/or pastoralists[citation needed] who were pushed into marginal areas as a result of migrations, economic exploitation, and/or violent conflict (see, for example, the Kalahari Debate). The result of their effort has been the general acknowledgement that there has been complex interaction between hunter-gatherers and non-hunter-gatherers for millennia.[citation needed]

Question: What caused these one-time agriculturalists to become foragers??

Answer: pushed into marginal areas