What causes the electromagnetic waves to prefer one dielectric over another?
Dielectric wall reflects waves. Reflection is complex because every time the wave meets a border of medium it gets partially reflected. Depending on wavelength, plate thicknesses, their materials and placements it's well possible that the radiation to one direction is stronger than to the other direction. The radiation is strongest to the direction where most of the waves (incident, reflected, reflected multiple times) travel with least phase cancellation.