What is the cause of voltage phase shift in a power system?
Electricity doesn't travel infinitely fast so, at some distance down a cable, the voltage will be delayed by a small amount with the delay getting bigger as distance increases. A fixed delay = some amount of phase shift.

I know this is for much higher frequencies but the dielecric material in a cable affects velocity of propagation of electricity at any frequency but, it's more noticable at higher frequencies in a lab using an oscilloscope: -



Basically it's telling you that the wavefront of a signal travels down a cable at speeds slower than the speed of light when the relative dielectic constant is greater than 1.

For a very long power cable you will see what appear to be phase shift along the cable but this is due to the velocity of propagation not being infinite. Here are the simple formulas that describe the speed of light and velocity of a wave down a cable: -



This is a useful site for learning.