Well it is a random thing with my girls.
It never happened with my first flock but has happened twice with my second and those girls were a year old in April this year.
It just amazes me when I find those ity bitty little eggs.
When we opened the first one it was just the white, no yolk.
The ones without a yolk can be formed by a hen of any age, though it is a lot more common with pullets coming into lay. Normally a yolk starts the egg laying process, but sometimes a piece of material sloughs off from the hen's internals and triggers the eggmaking system to produce an egg without a yolk.