I once owned a car that did this, too. Although it is unusual, it is not really a mystery.
Everything has a "resonant frequency" -- this is the natural 'ringing' vibration it exhibits if banged, or plucked, or twanged, or thumped. Whether it is a church bell, wine glass, grandfather's clock pendulum, suspension bridge*, or Ridgeline, all objects "ring" when struck.
If the windshield wiper frequency matches the RL's resonant frequency (or one of its harmonics) then the vehicle will begin to rock in time with the wipers.
(And, you know it's true because you read it right here -- on the internet.) :wink:
*
Tacoma Narrows Bridge - 1940.