I would guess that one of the big reasons that there are a "mad-rush of millennials" into select cities is that most of the ambitious, type-A personalities used

I would guess that one of the big reasons that there are a "mad-rush of millennials" into select cities is that most of the ambitious, type-A personalities used to try and pick a mate in the high density environment of a selective university. That is not nearly as common now but un-paired people still want/need a high density of potential mates that meet their minimum standards. Choosing what happening city to move to after college is now much more important instead of getting a job(s) and moving directly to a home selected for raising the kids.