Do you understand that if a = 0 the pressure gradient is pg? Well, if acceleration upward = g, then the fluid is exerted to both acceleration due to gravity = g plus acceleration of the fluid itself = g, for a sum of 2g. Think of it this way: if you are in a rocket accelerating upward at 9.8m/s^2 you will experience 2 "g's" of force acting on your body.
