External auditory canal exostosis occurs due to

Exostoses are multiple and bilateral, often presenting as smooth, sessile, bony swellings in the deeper pa of the meatus near the tympanic membrane.They often arise from the compact bone. Exostoses is often seen in persons exposed to the entry of cold water in the meatus as in swimmers and divers. Reference: Diseases of Ear, Nose and Throat, PL Dhingra, 7th edition, pg no. 118