Is this a new theme? Well it certainly seems so, although it is clearly based on the Fate Motive of the first movement.
The horns unexpectedly burst forth with the Fate Motive, accompanied by the strings which accentuate the first beat of each bar.
This rhythmic motive is played three times on the same note, and in a descending scale on the fourth time. It comes as a rather strong contrast to the mysterious opening of the movement.