I've noticed that some devices cause this and some don't. Last time I checked there was no way to prevent it.
I've had rental cars that did this...blaring out the exact first song, alphabetically, in my Music app Library (of the songs downloaded to my iPhone) when I plug into into USB.
I created a 12 minute track of complete silence...naming it "AAA Silence". I figured I'd notice, in at least 12 minutes, that the onboard audio system was playing it! It works for me.