Usually large eggs have double yolks...so it's because she released 2 yolks at once.
How long has she been laying?
Not uncommon with newly laying high production type birds.
This egg is way bigger than the normal. Usually I get extra LG eggs with double yokes. I'm going to crack it and see if possibly it could be a triple yoke. I've just never had an egg this large and wondered if this was normal for a leghorn?