Walter, the Akkadian Empire collapsed around 2100 B.C. primarily due to external pressure, specifically, attacks from hill peoples coming from outside Mesopotamia. Frequently in world history marauders will come into a settled area, cause a lot of chaos, and then turn around and leave. The pre-existing authorities in the region aren’t always able to pick up the pieces.
Other times the invaders will try to stick around and run the territory they’ve invaded with varying degrees of success. There’s no hard and fast rule on this . . . you just have to look at the particulars of the situation.
I hope this answers your question.