The shift from the bass bridge to the treble bridge is required because the bass bridge's fourth string G is the start of the lower tetrachord of the G scale. The player could go on up a couple notes (G – A – B), but the next note will be a flatted seventh (C natural in this case), because this note is drawn from the G tetrachord. This D major scale with a flatted seventh is the mixolydian mode in D.