[odysseus]

This happens because:

1. The original app that was playing (Podcasts, Spotify, etc.) got automatically killed for whatever reason (consuming too much memory, etc.)

2. The default app for audio playback is Apple Music, so if you press play (or if your car resumes audio), that is the app that starts up.

There is no way (at least right now), to change the default app for audio playback on the phone.

[cygned]

And that happens in the very moment when I plug in my phone? And even if, why that song? It’s not the first one. It’s neither the last one. And it’s always that one. I haven’t even downloaded it and streaming over mobile data is disabled.

[pwinnski]

There used to be an option to let Siri "help" you play the right music at the right time, which I foolishly enabled because I thought it might make things better. It would do things like suddenly revert to something I'd been playing days before, because Friday night I was in the car and playing it, so Monday morning when I got into the car again, I must want to play that again, right? Once I figured out that was why my phone kept jumping around to seemingly-arbitrary things, I turned it off, and now my phone plays whatever I was last playing 99.9% of the time.

I don't know where that option is, or maybe it's gone with iOS 13. It was annoying.