I have the same problem and I am not sure what causes it. Ever since the music controls have been added to the lock screen I have had a problem. Like many guys, I put my iPhone in my front pocket. I have found, almost daily, that after simply moving around (walking, sitting, standing), the music player will start playing whatever the active song was - basically somehow it unpauses. Arguably this would require that the home button be pressed twice and that the play button is also pressed - I get that. However, something is happening as it is more common than not. I don't even put anything else in that pocket anymore.
Frankly, it is very frustrating and after reviewing numerous posts in numerous blogs, this problem is not limted to me. When it does happen, I find that my iPhone battery runs into the ground - I would go so far as saying that this is a safety hazard. If I am stranded somewhere with no phone battery because Apple decide not to give me an option to turn off the "lock screen" music player function - I would go so far as argue that this is Apple's own negligence. A "lock screen" by definition should LOCK THE SCREEN. This is not a "lock screen " with exceptions.
My only other option to fix this issue is to empty the iPhone of the music library. At that point, I might as well get a competitve product that is not primarily a music player.