So there's actually three different components involved with this. Someone else has already mentioned the possibility of cars that just lie to your phone and say you pressed the play button because "well the user connected their phone they must want music".
The OS itself is also responsible for managing where that "play" command goes, and because this is a mobile device it also manages what apps are in memory, which one owns media playback, etc. If nothing is currently playing, it has to pick something, because you pressed the play button and you're currently driving down an overextended highway at unconscionably American speeds and can't be arsed to care about what app's play button needs to be pressed.
Individual apps can also grab or drop the media playback role at any time. Maybe that game has some background sync nonsense to send you a bunch of notifications, and whenever it gets woken up to do that the game engine it was written on immediately tries to start media playback because nobody tested it for background use.
Music's inconsistent behavior sounds like someone didn't implement state resumption correctly.
The underlying problem is that nobody owns the whole experience and this all is supposed to happen without projecting selection UI to the user. The phone just hears "PLAY MUSIC DAMN YOU" and makes a shitty guess as to what you meant.