iPad Air Headphones Going Quiet

If I leave the app while I'm watching a YouTube video, sometimes (but not always) when I go back the video is quieter. It doesn't change the volume setting at all, it just gets quieter. I have to unplug my headphones and plug them back in, and then it goes back to normal volume. This also happened once between songs on Spotify. What's going on? I tried buying new headphones but they have the same issue.