Ours aren't laying yet but I read that an wrinkled egg can be caused if it gets cracked as it's forming, and then resealed by more calcium, you get a wrinkly but intact egg. From "Storey's Guide to Raising Chickens", I think?
its cause by bruising or breakage while the egg is forming happen's alot.i have even had a egg inside a egg before,thats from it starting down then getting pushed back in the ovum then a new eggshell grows around the old one.