Music turns on while on call

I recently got an Apple Watch 4. While I’m on a phone call in my car (using Bluetooth to the car) the phone will seemingly randomly start playing music. During my phone calls. It looks like the watch is turning it on but hitting pause on the watch seems to work for less than second and it turns back on. Am I giving it some sort of voice or movement command to make it turn on the music? How do I make it stop?