the causes of Fading of signals
In communication systems, fading refers to a specific kind of attenuation which is highly frequency and time dependent. Other types of attenuation such as loss on transmission lines, path loss in radio transmission etc. do not change rapidly with time or frequency, at least not within the bandwidth of interest. On the other hand, fading varies strongly with time and frequency and can - unlike other sources of attenuation - result in a very large frequency-dependent attenuation. And this is caused by multi-path propagation.

As a very simple example, consider two paths. One direct path and another path with attenuation \$a\$ and time delay \$T\$. The squared magnitude of the total transfer function is

$$|1+ae^{-j\omega T}|^2=1+a^2+2a\cos(\omega T)\tag{1}$$

From (1) you can see that the frequency dependent attenuation can be quite large if \$a\$ is near unity. This example is of course overly simplistic because it is time-invariant and because there are only two paths. Fading is an important phenomenon in mobile radio and can be mitigated by using diversity techniques, i.e. by combining several channels and hoping that not all channels are affected by fading at the same time and in the same frequency band.