The most common reasoning I’ve found for the bow in the Nicene Creed at the description of the incarnation is that the bow is a physical acknowledgement of the importance of this event. If that’s the case, why do we not bow at the crucifixion or resurrection? These seem to be of equal or greater importance to us.
When we bow in respect to the Incarnation, we are bowing for the Crucifixion and Resurrection as well since it was only as the result of the former that the two latter took place.
Fr. Vincent Serpa, O.P.