Naturally curly hair occurs due to hormonal changes in the body.
When a person develops naturally curly hair as they age from baby to toddler and on up the person with naturally curly hair has hair follicles that are asymmetrical instead of symmetrical.
When a person has symmetrical hair follicles the persons hair will be straight but if the person has hair follicles that are asymmetrical the persons hair will be naturally curly.
Certain cancer medicines that a person could be taking can also change a persons hair from straight to curly too.