In the natural world, albino (no pigment at all) & leucistic (a rarer condition; no pigment in body covering. these individuals will be white with dark eyes) individuals have recessive genes for pigmentation. A single indivudual may pass on some of these recessive genes into the population; but they'll will soon "fade" as their expression is masked by the more dominant genes for coloration.

Often times, there are multiple genes for coloration and the more "white-lighter" ones an organism has, the lighter its color will be. Hair color in people is a good example.

If the number of "white" individuals is high enough, that characteristic will express itself more often than in a population with lower numbers or one "white" gene source.

From the sounds of it and the image posted, it looks like there's 2 possibilites goin on in your area's turkey flocks. One, there was a large infusion or a steady small infusion, of white individuals (domestic or naturally expressed mutations). Two, the light to white color is the result of a problem somewhere in the genes that code for the pigmentation (some possibilities: don't make the color or make the wrong color) in the affected individuals.

That the mutation is so evident in many areas leads me to assume that you may not have much of a predator problem there, which is in general a good thing turkey-wise.