One of the more compelling factors I've heard of for the fall of the Roman Empire is their tax system for agricultural land. Essentially it's a per-acre tax regardless of the productivity and ease of access, with the obvious problem of marginal land getting abandoned because the tax wasn't worth paying.

I read that a lot of the invading tribes were treated as liberators from the Romans for that very reason. Malthus was in full force in the era, so you had edge-of-starvation villages next to productive land that could be cultivated but wasn't due to taxes, and the barbarians rolling in meant you could do that now.