I made the mistake of triggering Siri in the car to play an album I owned…and it signed me up for a one week Apple Music trial subscription instead. The switch was seamless until I saw the accompanying email. And there was no confirmation via the interface (visual or verbal). I still have the listing in the Subscriptions area of iCloud after I made sure it was cancelled. It hasn’t happened again, which I suspect is because of the listing.

A possible cause is the Siri-enabled button on the remote that can trigger music requests.