1. Egg yolk is generally due to the incomplete development of laying hens, resulting in poor egg development and no yolk. Another reason is that laying hens themselves suffer from diseases that affect reproduction, so they all give birth to such deformed eggs. Although this kind of egg does not carry diseases, it is best not to eat it.

Or man-made

2. When laying eggs, there will be no yellow eggs, because chickens mistake blood clots or parasites for egg yolks and wrap them in eggshells.

There is an explanation in the book "100,000 Why".