Since Roman was expanding and it didn't have control of their lands and couldn't govern most of it. It kept expanding until the western part of Roman was conquered by Barbarians.
I believe the answer is: the formation of feudal kingdoms
Feudal kingdoms refers to a kingdom which is ruled by several lords who owned the lands and had the right to hire Vassals for their disposal in the kingdom.
When roman empire fell, the high status people from the roman empire separate themselves into several territory and bring their military influence with them. They established ownership to the residual land of the empire, and lead to the creation of feudal system.
