If it's not chimerism or passable genetics, it would probably be that the genes that block the production of the darker pigments have suddenly malfunctioned and that these gene are more prone to that specific type of mutation due to some combination of environmental and genetic factors.
The only way to figure it out is to breed'em I guess.
The thing with the paradoxes occurring spontaneously from normal to heterozygous isn't that strange to me... of course, it could be possible that the normal was actually het, but it is also possible that the egg/sperm itself that was first produced via meiotic division in the mother/father had a spontaneous mutation when it was being formed--remember, all inheritable morphs (other than the wild type) and genes were once mutations as well--the only difference is that they can be passed down to next generations.
One's an inheritable trait mutation, the other isn't.
So I can actually understand that occurring.