It happens sometimes. Pushing out a big egg is no easy feat. Sometimes it's indicative of a new layer or slight damage to the reproductive tract. Sometimes my older birds will have this happen for a few days and then have a permanent deformity in their eggs afterwards, indicating that there was a minor injury during lay that healed with a bit of scar tissue.
It's usually not a big deal but keep an eye on your girls for prolapse or other problems and make sure they have lots of good food, water and free choice calcium.