Same happens to me with a Hyundai Tucson under 8.1.x ; whenever I plug the phone it starts playing, and can't pause. Siri also won't work since as soon as it pauses the music, it resumes playback and interrupts Siri.
However at some point it did fix itself; it was something like I connected the phone while in the middle of a call after having done a sync, and then turned the car on and it did not initiate playback (since I was in the middle of a speakerphone call), and from then on it worked fine, did not do the automatic playback anymore. Everything worked as it should for weeks, until I upgraded to 8.1.2 and then it broke again. So it's some kind of setting or "cookie" that'd be stored either in the phone or in the car. Haven't been able to replicate the "fix" again.