Why the rocks in the fence?

It´s some sort of warning system,
if someone attempts to climp or cuts the wires the rocks would fall out and
make a noise wich would most likely alert the Southkorean Borderguards.
A similar method was used by U.S. troops in Vietnam. They used to hang empty foodcans with a bit of wire inside (a makeshift bell) to strips of barbed wire,
when an infiltrator would sneak to a base and try to cut assault lanes in the wire it would alert the G.I.´s on guard duty.

The white-painted rocks were inserted into the fence to aid night vision - they'd stand out at a much greater distance to enable anyone wearing NV goggles to better judge if someone is on one side of the fence or another (which can be that person's life or death in that situation), as well as making sure that someone running with NV on stands a better chance of not running right into it as the dull and thin framework of the fence would be extremely difficult to see :)