What is a likely cause of irregular fading of signals propagated by the ionosphere?
Why A signal refracted by the ionosphere can reach you over more than one path: different hop counts, different layers, or slightly different launch angles. Those copies arrive with slightly different travel times, so their waves sometimes add in phase and sometimes cancel, and the result is signal strength that rises and falls irregularly. This is multipath fading, and on HF it produces the familiar slow flutter and occasional deep nulls.
Watch out Faraday rotation is real, but it twists the polarization of the wave rather than shifting frequency, and it is more of a satellite-path concern; intermodulation is a receiver or transmitter mixing problem, not a propagation effect.
Fading = the same signal arriving twice by different roads, sometimes helping, sometimes cancelling.