Whenever I have gotten weird eggs they have been layed most often by a pullet.
It's definitely more common when they lay their first eggs. I have several "ameraucanas" which are actually easter eggers plus several other breeds. It seems like they start laying very large eggs and then settle in to nice large eggs, whereas most of the other breeds start small and then get larger, and they all have tendencies to lay eggs with no shells on their first tries.