Sometimes small pieces of tissue travel from the ovaries down the oviduct, her body treated this as if it were a yolk. This must not have gotten expelled from the uterus or shell gland as it's also called and her body kept adding layers of what would have been the white of the egg. It spent a lot of time in the shell gland cooking. Many times this happens because she has an oviduct infection. If she continues to lay these, a 7 day round of Amoxicillin, 250 mg 3 times a day for 7 days, will clear up the infection. Use probiotics after treatment.