rhetorical question used by teenagers. "turn up" is the act of getting drunk and high and being reckless so "turn down" would mean sobering up. turn down for what is really saying i am fucked up and will continue to be all night no matter what. the only appropriate answer to this question would be "nothing"
Johnny: turn down for what ?!
Ben: The cops and my dad.
Johnny: You're a pussy! TURN DOWN FOR NOTHING