Besides the distraction of music in general, there's a couple other factors going on. One is that parking is a relatively less practiced skill than driving, so

Besides the distraction of music in general, there's a couple other factors going on. One is that parking is a relatively less practiced skill than driving, so it should naturally require more concentration. It is just plain awkward as well. Second, unless your stereo system automatically adjusts the volume level based on speed, the music will seem louder than when you were driving.