The most frequent cause of entropion is natural ageing which results in weakening of the eyelid muscles and laxity of eyelid tendons. This can make the eyelid unstable and prone to turning in.
Other causes:
Eyelid trauma or surgery
Conjunctival scarring
Excessive eyelid rubbing causing eyelid laxity
Infection (worldwide Trachoma is a common cause of entropion in developing countries)
Entropion can, rarely, be present from birth