I was led to believe this was due to stress, disease and/or over crowding in hens. Also sometimes because of a defective shell gland.
I found this too
escription
Misshapen eggs are those whose shells differ obviously from the smooth, ‘normal’ shape. They include eggs with shells marred by flat sides or body checks (ribs, wrinkles or grooves), and eggs that are too large or too round. Flat sided eggs are discussed on page 37, and bodychecked eggs.
Incidence
The incidence of misshapen eggs varies considerably depending on how severely the eggs are judged. Normally, up to 2% of production is downgraded due to these faults. The incidence of misshapen eggs can vary with the strain of bird, but they are most often produced by pullets coming into lay, or hens late in lay. often as a result of double ovulation.
Cause Control
Immature shell gland Delay onset of sexual maturity 1 to 2 weeks by controlled feeding during rearing.
Defective shell gland Cull birds which persistently produce such eggs.
Diseases, e.g. infections bronchitis Follow an effective vaccination programme.
Stress, e.g. frights and disturbances To avoid frightening birds, minimise human activity in and around the shed. Increase shed security to stop other birds and animals entering the shed.
Crowding Avoid overstocking.
