I assume that you are asking about Norman Yoffee’s chapter in the book Questioning Collapse, which he edited along with Patricia McAnanay. In this chapter, Yoffee presents an argument about the collapse of the Assyrian state that, unlike the argument presented by Jared Diamond in Collapse, has nothing to do with environmental degradation.
According to Yoffee, the main reason for the fall of the Assyrian state and its inability to regenerate was what might be called overreach. In other words, it was the imperial policies of the Assyrian kings that caused their state to collapse. Yoffee says that the mistake that the Assyrian kings made was to bring in too many non-Assyrians from conquered lands. He says this meant that much of “Assyrian” society was made up of people who had no connection to Assyria or its rulers. What this meant was that, when the Assyrian rulers were defeated, there was no one left to regenerate Assyrian society. In the past, Assyrian society had really been Assyrian. Therefore, when rulers and elites were defeated, a new set of Assyrians would rise to the top and become a new elite that was still Assyrian. But now, the bottom layers of society were not really Assyrian anymore. When the top layers were defeated, no new elite was created and the Assyrian state did not regenerate.