Double yolks happen when the hen first starts laying eggs. I've also heard that it is caused when the hen is kept laying all the time without a break.

There was a big article about factory egg farms and how double yolks pointed to stress and mistreatment, as I recall McDonalds was pushing for better treatment of hens from it's suppliers. It is a natural phenomenon.

http://www.poultryhelp.com/oddeggs.html
http://en.wikipedia.org/wiki/Egg_yolk#Double-yolk_eggs
http://www.examiner.com/bakery-in-na...rare-after-all