I use poweramp and I do not have this problem with my Sonata. I suggest if you are using the stock music player, maybe look into downloading another player. Or go through your settings to prevent music from automatically playing once connected.
I have the stock player and poweramp. As long as I am listening to the radio no problem. If I set the radio to receive the signal from the Droid X the music player always begins playing. Sometimes the stock player or it could be poweramp. If I close the music player and am listening to a radio feed from the Droid all is fine until I turn off the motor and restart, then the music player begins first all by itself. If I switch back to something like Sirius then back to the Droid then the music player starts again. I have been through every setting in the music players. Can't figure this one out.