Hi there,
I’ll bite, Looks to me as if the USB is connected the small voltage bias provided by the Zener Diode is pulled lower and Shuts down the path to GND for the 3.3v path.
With out it (usb) plugged in then the mosfet is able to sync the current and complete the path to GND.
my .02
Others can comment on more.
HTH
GL 
PJ
