This happens when an egg is formed in the hen's uterus, but she is unable to lay it. The straining with unsuccessful attempts to lay can cause the oviduct to partly protrude. Other fowls cannibalise the affected hen and she may die.
The condition can result from a large egg, growths or tumours in the oviduct, diseases affecting the nerves or mucus secretion in the oviduct. An experienced person can help remove the egg by lubricating the oviduct and breaking the egg.