The defensive strategu of S. Korea and U.S. until mid-70's was to retreat to the south of Han river and regroup and counterattack, upon N. Korean invasion.
Then it was changed to a new strategy of holding the position near DMZ and push them back without any retreat.
Now America changes its strategy again. This time pulls her ground force from DMZ to the safer area to the south. It will skip the part of retreating and regrouping altogether. American troops appear to be positioned as a main counterattack force while the S. Korean ground force will do the work of wearing down the initial thrust of N. Korean troops.
This is how it looks on the surface. But under the current context, the relocation could be also for the pre-emptive attack if it becomes necessary. As I said, it is an implied threat to N. Korea, which will be duly noted by all players, including N. Korea.