I changed the brakes and turned the front rotors on my 07 G35 because l felt pulsing or vibration in the front end when braking. The shop said the rotors were not warped. I did not turn the rear rotors.
Now about 3K miles later it's happening again.

What's causing this?

I don't drive at fast speeds and most of my driving is city or on hills at 25 MPH.