Why is USB D+ tied to VDD (for PCM2707 at least)
It is part of how USB negotiates the speed at which the host and device plan to communicate, as well as allowing for presence detect of a device being plugged in.  Basically, the host has a 15K pull-down on both D+/D-, and the device has a 1.5K pull-up on one of the 2 lines to indicate full or low-speed.  You can read more at the following link: USB in a NutShell.