The main cause of ear popping is change in air pressure. This can happen for a variety of reasons, from planes frequently shifting their altitude to a train entering a tunnel - thereby compressing the air.
Colds, allergies, sinus infections and even enlargement of the adenoids can also cause excessive ear popping
through Eustachian tube dysfunction. Sufferers of Eustachian tube dysfunction
are discouraged from flying altogether or, at the very least, should use over-the-counter medication to help clear these lest symptoms are exacerbated during flight.