There is a explanation, maybe? The Urban density was rural farmers moving to cities after the agricultural owning elite kicked them off their land and was using slave labor to grow crops.

One of Romes greatest conundrums was figuring out who to fuck over with rural/agricultural reforms. Do they screw over the rich and give it back to the poor? Or do they let the rich land owner continue to work these fields. That is a VERY small summary of the situation regarding this but I think that may be a root cause, at least in the Italian peninsula.