Why is harmonic voltage flowing to the source
Start with non-linear loads creating distortion. Very commonplace such as in a bridge rectifier. The capacitor gets charged only at the top of the rectified wave (see the black trace below): -



This results in an AC current that looks somewhat like this: -



Now this can be analysed and broken down into harmonics. Those harmonics are representations of the non sinusoidal shape of the waveform and as such, it would be daft to think of those harmonics individually being taken from the source but, it is reasonable to think of them harmonically when analyzing how these harmonics may get re-shaped as they pass down the cable.


  how can these Harmonic Distortions flow back from the load ?


There is no difference physically between a signal that is created from summing harmonics AND the original signal that was harmonically analysed. So it doesn't matter which "model" you choose - both have the same influence back at the voltage source.