I have just recently began having this problem as well. At about the same time, my apple watch battery has been draining EXTREMELY fast (stone dead within hours). I would assume that it is related to the problem somehow. Shortly before all this happened I had began using Apple Music. Its interesting that when my phone randomly begins playing music, its ONLY my downloaded apple music.
Even more interesting, I just found out a few minutes ago that my top listened to apple music had been syncing directly to my watch's storage. When I looked at what was on there, they were the EXACT songs that my phone had been playing.
I'm not sure if this problem is related to the people in this thread who don't own apple watches or use apple music, but the symptoms appear to be the same as mine. I have disabled siri/voice command on both my phone and watch, shut down all background updating of apps, and disabled music syncing to the watch. Hopefully this fixes my problem.