What is the effect of not grounding a power supply to a servo?
Assuming your Rpi3 is connected to a regulated USB power source, both the grounds, the servo power supply and the USB power source need to short so as to give a common reference signal for the whole circuit to work on. 

Your reference(PWM) signal is coming with respect to the Rpi power supply ground but the servo is being supplied with a different 'common refernce signal'. Hence, the servo is not in 'sync' with the Rpi common reference and therefore the servo is still being fed with a supply but with a different ground and hence you see the servo ticking.