Why might your ears pop when you travel up a mountainside or fly in an airplane?

Your ears pop in airplanes because the air high above the surface of Earth is The High pressure when you are going up into the air

The pressure up in the plane or on a mountain is lower than the pressure at ground level. Since the air pressure is lower, that means some of the trapped air in your inner ear must escape to stabilize the pressure (so the pressure in your ear is equal to the lower pressure outside). As a result, your ears 'pop' to let out that trapped air in your ears and ensure that the pressure in your ear is equal to the pressure outside.