The only thing I could think of is that Rdio or any ad-network is sharing information with a 3rd party and that 3rd party pushing the info to Spotify. The other way is through your social network like activity. Did you "like" the band on Rdio?
Why would rdio make this information public? It seems like really valuable information that they are just freely allowing to get scraped
I didn't "like" the band, I just listened to their stuff normally. Since Spotify runs natively, I wonder if they have a way to access browser information
I say spend some time trying to re-create it. Maybe some activity within Spotify itself triggered the email.