Random? What are you talking about? You've never seen that amazing iPad app that the TSA officers use to tell you which line to get in? You know... the one where they tap on the screen and a large arrow tells them whether to send you to the left or right? And for which they probably paid tens of thousands of dollars (maybe more) to have built? Wish I had gotten that contract! :-p
That just picks whether or not you get PreCheck if you weren't already on the list. It doesn't necessarily determine if you're in a metal detector or MMW line (though PreCheck lines only have metal detectors).
