I think it is something to do with the playlist, if there is such a thing, as it is intermittent.
I've noticed this playing-at-the-same-time occasionally, as mentioned here, in my front yard.
In my back yard I get extended periods of no music, then it will suddenly start playing something again.
This all started ~4 days ago.
Fascinating! I agree with BlackWolf - it's relaxing in an odd sort of way. I may have to keep that link bookmarked just to hear that blend now and then...