Some points of my recent response to a related question might (partly) answer yours. Here is the specific bit on the cause (follow-up questions are of course welcomed, since it was peripheral to my original answer):

« What caused it? You seem to be looking for overarching social explanations; unfortunately, I don't think they exist. The concept of empire is an incredibly flexible thing; the ERE can be said to have remained truly “imperial” for at least 500 years after the fall of the West, and it is obvious that its social structure changed a lot during this very long period. To explain what happened in the 5th century West, there's really no way to avoid purely military and contingent factors (for a discussion of what made the West vulnerable, see this post for some leads). Certainly, some historians have tried to find global explanations; some are convincing enough to be integrated as subsidary factors in more general models (e.g. the fact that latifundia were more common in the West, and that the concentration of landholdings made tax evasion more prevalent).

If you are looking for a sweeping perspective, however, I would suggest that you look at the way empires tend to influence neighbouring peoples, something that has been thoroughly explored by people like Peter Heather. Imperial-“tribal” border zones tend to set off a series of processes—the military threat of the empire creates a new need for political unity; imperial attempts to undercut these attempts by choosing particular client tribe (to divide and rule) leads to a concentration of wealth in a few hands, and thus, ironically, to an eventual centralisation of power; and this gradually builds into a very dangerous situation. »