Why As a mobile station moves, signals arriving over multiple paths alternately reinforce and cancel, so the received signal strength rises and falls many times per second. At VHF and UHF the wavelength is short, so the vehicle passes through those peaks and nulls very quickly, producing a rapid flutter or buzz on the signal. The name comes from the sound, like the rhythmic flicker you get looking through a picket fence as you drive past it.
Watch out The cable TV answer describes leakage producing evenly spaced carriers, which is a different problem entirely, and the ground system choice is describing radials under a vertical.
Picket fence = moving car + multipath = fast flutter. Think of driving past fence slats.