Why do your ears pop when you take off in an airplane?

You may have Exercise-Induced Eustachian Tube Dysfunction…

Exercise increases blood supply to all parts of the body. Along with this increased proliferation of blood, comes tissue engorgement...this is why your skin becomes red or flushed. When the tissue engorgement is around your eustachian tubes -- those tiny ventilation tubes that run from your middle ear space (the area on the other side of your eardrum) to the back of your throat -- it may cause the tubes to malfunction.

If the eustachian tubes clog, your ears may feel full / stuffy, and your hearing could be a bit muffled, like they need to "pop". If the e-tubes lock in the "open" position (they are normally closed and only open briefly when we swallow), you may hear your own voice or an echo.

As long as this exercise-induced eustachian tube dysfunction resolves after you cool down, there is no need to be concerned. Unfortunately, there may be little that you can do to prevent this from occurring, other than reducing the intensity of your exercise.

I've also heard it can come from being dehydrated or from breathing into your nose and mouth at the same time while exercising.