Now I'm no expert in late Roman economics, but its my impression that the increasing burden of military commitments led to brutal taxation which all but wiped out the Roman middle classes. One consequence of this was an ever widening gap between the rich and the poor.
In a British context, this could have meant that small independant citizen-farmers and village economies were put out of business and forced into tenancy agreements with the super-rich landowning class. As a result, the wealth of the province became concentrated into the hands of a very small number of magnates. These people chose to flaunt their fortune through the building of palatial villas for themselves.
Since recession always hits the small businessman hardest, I'm inclined to think the growth of "villa culture" in the fourth century actually indicates a decline in the net wealth of the province, and an increasing division between the aristocratic class and the common citizens, who became forced into a condition that was little better than slavery.