What is a likely cause of irregular fading of signals propagated by the ionosphere?

When a radio signal travels through the ionosphere, it often splits and refracts along multiple paths. Because the ionosphere is a dynamic, constantly shifting layer of charged particles, these different paths have slightly different lengths and travel times. When these multiple signals finally reach the receiving antenna, they recombine. Sometimes they arrive in phase and strengthen the signal, but often they arrive out of phase and cancel each other out. This continuous phase shifting creates the irregular fading effect commonly heard on HF bands. Distractors like "intermodulation distortion" refer to equipment issues rather than propagation, and "thunderstorm interference" typically causes loud static crashes rather than continuous irregular fading.