Hypothesis: the Roman Empire just became too complex to afford sustaining itself anymore.
According to Tainter, societies become more complex as they try to solve problems. Social complexity can include differentiated social and economic roles, reliance on symbolic and abstract communication, and the existence of a class of information producers and analysts who are not involved in primary resource production. Such complexity requires a substantial “energy” subsidy (meaning resources, or other forms of wealth). When a society confronts a “problem,” such as a shortage of or difficulty in gaining access to energy, it tends to create new layers of bureaucracy, infrastructure, or social class to address the challenge. Eventually, this cost grows so great that any new challenges such as invasions and crop failures cannot be solved by the acquisition of more territory. At that point, the empire fragments into smaller units.
Joseph Tainter, The Collapse of Complex Societies