Why are my chickens eggs soft?
By soft I presume that you mean that the egg shells are soft, thin or just have a membrane type outing.
If it is just a once off or occasional occurrence it is most likely nothing to be concerned with, however if it does continue I advise contacting your veterinarian.
The most common cause of soft-shelled eggs is to do with an inadequate diet. Make sure that your chickens are receiving enough calcium and Vitamin D3 in their diet. Most commercial feeds are well balanced and you can offer your chickens a variety of kitchen scraps like vegetables that are high in calcium and vitamins to help boost their intake. The addition of grit to their feed my also help for strong egg shell formation.
A shock or stress can also cause hens to lay soft eggs, however this is usually just for one or two days and then things go back to normal.
Other causes of soft shelled eggs can include diseases such as Newcastle Disease (must be reported to the authorities) bronchitis or other viral diseases. If you suspect that illness or disease may be the cause please contact your vet immediately.