You'll find all kinds of eggs that are strange. Nothing to worry about. One of my chickens lays eggs that are all squishy like a water balloon but it's rare. Just a blip in their system but as the other BYC member stated make sure they have crushed oyster shell at all times.
Are you providing them with crushed oyster shell, and are they eating layer feed? I would recommend purchasing some crushed oyster shell to supplement their calcium intake. Or, grind up some clean egg shells into a powder and feed them that. I use a coffee grinder to create a kind of eggshell powder, and then sprinkle it on their feed. Make sure that you aren't feeding them too many treats, as that will unbalance their nutrient intake.