I solved a mystery recently, an Icelandic folk mystery.

Every time I plug my iPhone into any car other than my own car, Icelandic folk music stars playing.

Specifically, the song Á Ferð Til Breiðafjarðar, by Steindór Andersen and Sigur Rós, from the album Soundtrip Iceland starts playing.

This happens in Lisa’s Nissan Rogue. It happened in the BYD electric car we rented in France last year. It happened in the Citroën we rented in Italy this year. It happens enough that Lisa, L., and I can recite the first verse almost by heart.

But why? Why this song?

Here’s what I figured out: for reasons unknown, my iPhone, connected via Bluetooth to a non-Carplay-enabled car stereo system, defaults to playing Apple Music.

The music it defaults to playing is from my Apple Music library — it’s not streaming music, per se, but rather the albums that ended up, through various generations of iTunes and Apple Music, being ingested into my library from CD.

And, specifically, it defaults to playing the songs in the library, in alphabetical order of song title. The Icelandic letter Á happens to sort first in my library, so that’s why it plays first.

Á Ferð Til Breiðafjarðar is always followed by Abraham’s Theme (From Chariots of Fire), and so on, through the As, the Bs (Baba O’Riley, by The Who), the Cs (California by U2) and Ds (Das Berliner Requiem). And so on.

This is a motley collection of music, from different eras of my life, from different sources. I’m not certain I know how to get new items into this library, or take old items out. And I haven’t yet figured out how to change Apple Music’s default behaviour.

But I’m happy to have solved the mystery.