Although that supposed trend seems to be exaggerated--at least in the US. According to this piece http://fivethirtyeight.com/features/why-millennials-are-less... for example, the actual trend seems to be that a fairly narrow slice of demographics (25-34 with college degrees) mildly prefer living in a few specific dense urban environments like Brooklyn.
Overall there is something of an urbanization trend (especially in emerging economies) but it's not as pronounced as it's often made out to be.