I hear it here a lot in NYC, but I know it's a national thing. Their voices borrow something from the old 80s-style valley-girl speak. And of course their declarative sentences end in a question-style tone raise -- "uptalk" I gather it's called.
But what really gets me is that though in some ways their voices are Valley Girl 2.0, it's valley girl not so much high-pitched or low-pitched, but rather
strangulated in the throat. They all sound like disgruntled ducks. Remember Batman? They all sound like the daughters of the Penguin -- QUACK QUACK QUACK QUACK QUACK!
So where do they get this voice from? Its high obnoxion certainly cuts through background noise like a chainsaw, so it may have, in part, been an evolutionary development out of the high-decibel ambiance in the bars and restaurants of today's upscale urban neighborhoods. I also sympathize with today's young women, and imagine perhaps that the disgruntled-duck voice may be their best attempt at a vocal "Third Way" -- a 21st-century nasality that is not any kind of frou-frou old-style "feminine" thing, but is also, in its Paul-Lyndesque qualities, not exactly typical of the standard male voice.
So where do they get that voice? :shrug: