"I didn't know Australian Shepherds were chimeric."
I don't know it for absolute certain, either. But, there are two causes for this that I am aware, which would be eye injury or chimerism, and I don't think every Australian Shepherd I've ever seen had an eye injury. Maybe there are other causes, but I'm not aware of them.
I think it's just a genetic trait since they can breed for it. I did find this snipette:
"It is told, that Native Americans referred to the dogs with mixed eyes as "spirit dogs", and considered them a gift and of great honor."