Because of the uncertainty principle there's always a probability that the electron will change it's state. so if we take a set of base states where each state x corresponds to the electron being at atom x then if atom x has a "hole" (missing an atom) there's always a non zero probability that an electron in the state x-1 will go into state "x" and then the hole will have moved to the left. (the same thing applys to electrons at other atoms)