Why A ground conductor is not automatically at zero RF potential. If the wire or strap running to the ground rod happens to be near a quarter wavelength (or odd multiple) at the operating frequency, it acts like a resonant stub: the far end is a low impedance, but the equipment end presents a high impedance, so RF voltage appears on chassis and enclosures. That is why you can get RF burns or erratic equipment behavior even with a 'good' ground, and why ground leads should be kept short or bonded in a way that avoids resonance.
Watch out A ground loop is a different problem, caused by multiple ground paths of differing potential creating circulating currents (often hum), not by resonance in a single conductor.
Quarter-wave ground lead = hot chassis. Low impedance at the rod means high RF voltage back at the rig.