What causes selective fading?

Selective fading is a specific type of fading that occurs when different frequencies within the same radio signal experience slightly different propagation conditions. Because a modulated radio signal spans a range of frequencies (bandwidth), the varying frequencies can take slightly different paths through the ionosphere. These different paths cause the various frequency components to arrive at the receiving antenna with phase differences. Some frequencies within the signal's bandwidth might arrive out of phase and cancel each other out, while other frequencies within that exact same transmission arrive in phase. This results in only parts of the signal fading at any given moment, which severely distorts the audio. It is inherently caused by phase differences between components of the same transmission, not by large-scale ionosphere height changes or time differences between stations.