Cecil, W5DXP wrote:
"What phenomenon of physics causes the energy and momentum in that wave
to reverse direction?"

It must keep moving and if it can`t go somewhere it goes elsewhere.

Shorts and opens are a way to control current which in its magnitude,
phase, and path produces fields.

In a transmission line or wave guide, waves are guided. In free-space,
waves move away from the source.

In a small single-turn loop, the current everywhere within the loop is
very nearly the same. A complete null is achieved on the axis of the
loop (see 3rd edition of Kraus` "Antennas" page 204, Fig. 7-7).

The loop`s null results from equidistance along the axis srom corrent
and fields which are in opposite directions. The fields are moving in
the same directions but because of their opposite polarities add to zero
along the axis.

So, to opens and shorts, equal and opposite radiation fields can be
added to the list of things which cancel energy traveling in a certain
direction. When a fixed rate of energy flow must be maintained,
cancelled energy must emerge in the noncancelled directions. It`s energy
conservation.

Best regards, Richard Harrison, KB5WZI