I don't know the answer but I do know that as a kid I would often find numerous Mantis egg clusters in very close proximity to each other. Sometimes side by side. Maybe when females see another laying eggs it triggers the instinct for them to lay eggs. Or maybe they just naturally seek out the same type of conditions that they feel will be a good place to lay. I would imagine that in nature all Mantis females lay eggs at about the same time in tune with the season.