This is the sort of situation that invites a dork like me to trot out my theory. It is much simpler than one might expect from the blurry junk pile of diverging trends that youth, in the memory of extant books and commenters, have picked up and soon enough tossed aside where cultural historians come along to force them into palateable rationales: The stronger that school-of-fish-turning-on-a-dime behavior, the less secure the creatures that are exhibiting it.
Herd behavior is an adaptation to the sense of vulnerability.
The confidence for genuine individuality generally comes to us much later than the urge to stake out an identity. Being human is not easy. Each style-as-badge a person adopts makes sense to them in the context of the moment of adoption. Being somebody recognizable but not exposed for being too different is a compelling consideration at a certain point in your life.