Sometimes I think that just happens, even with fresh eggs. Maybe it's something odd about the way the hen's body makes the egg that day? If you get a particular kind of weird egg regularly, you might just have one hen that makes eggs like that.
Other times, that can happen because the egg that has been shaken around. People who order eggs for hatching will complain when the eggs arrived with the yolks broken and mixed with the whites even though the shell isn't broken: in that case, the eggs are fresh, but the Post Office jostled them too much. If the eggs get carried around for a while in someone's pocket, or shaken by an excited child, that might cause a similar thing (I've carried eggs in my pockets without having the inside get scrambled, but I wasn't running or jumping when I did it.)
Yolks do become weaker over time, and more prone to breaking when you crack the egg open or even when you just wiggle it a bit. This happens faster when the eggs are stored in warmer temperatures. For example, it happens in eggs that spent one or two weeks in an incubator without developing chicks (infertile eggs.) I think it can happen to eggs stored at other temperatures too, but it usually takes much longer.
I don't know for sure what is happening with your eggs, but maybe my ideas will help you or someone else to figure it out.
