Music volume will randomly increase/decrease while headphones are in

While I am exercising with my iphone 5 (version 8.1.1) and apple earbuds, the volume of my music will sometimes randomly increase or decrease to the extreme (i.e. decrease to no sound, or increase to max volume). Why does this happen?

This has happened a few times with 2 different sets of apple headphones, and seems to be a malfunction. I am not accidentally hitting the volume buttons on the phone or earbuds, and I don't have any special settings for automatic volume change. Please help!