Music auto-plays when I plug into the headphone jack.

Whenever I plug headphones into the jack the phone starts playing music using an unknown player. The only music player I use (on purpose) is Google Play Music, but this music that starts on its own isn't from that. I can open Google Play and see it's got a different song cued up and if I hit play it stops whatever song was playing and plays the correct one. No other music player shows in recent apps, and I haven't even opened another player in months. Not even sure what other players I have. Nothing shows in the notifications, either. Oh, and if I don't interrupt the music by using Google Play, the music stops as soon as I unplug the headphones.

This weirdness only started a few weeks ago. Any ideas what I need to disable to stop this?