Final answer:
The precise reason for the Azawagh Arabs' settlement in Tassara, Niger, in Popenoe's ethnography is not detailed in the provided text, but it can be inferred that historical patterns of trade, Islamic expansion, and political conflicts influenced settlement behaviors.
Step-by-step explanation:
The main reason that the Azawagh Arabs portrayed in Popenoe's ethnography recently settled in Tassara, Niger, is not explicitly mentioned in the provided excerpts. However, the historical context given in the references suggests that the patterns of settlement in the Sahel and Sahara regions, including places like Tassara, have been heavily influenced by a mix of factors including trade, political dynamics, and environmental conditions. Groups like the Azawagh Arabs, who are part of the wider Berber (Amazigh) populace, were historically nomadic or semi-nomadic, engaging in trans-Saharan trade and sometimes migrating based on political or economic pressures. These historical pressures include the expansion of Islamic empires, the desire to avoid enslavement which led many Amazigh to convert to Islam, and the ever-changing political landscape exemplified by recent conflicts and demands for autonomy.