Facebook app lowers volume of music

If I'm listening to music on my 6 Plus (on the built in music app of course) and then open the Facebook app, it sometimes lowers the volume of the music. What's even more random is that if the facebook app is open in the background and I'm listening to music with the screen locked, it can also drop the volume down! So I'm walking down the street and suddenly the music in my headphones drops to about half volume as if I've received a notification. I have most notifications turned off for Facebook, so only get the little red bubbles with the numbers in to notify me.

What on Earth is going on?

Thanks for reading.

Dave.