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

Suppose you have a 50-ohm coax that has a 25-ohm resistor across its
load end. The incident wave traveling on the coax has too much voltage
and too little current to supply the 25-ohm load.

As the load forces the incident voltage lower, energy is conserved by
transfer of energy from the E-field to the H-field. Voltage generated in
this energy transfer between fields is reversed in phase from that in
the incident wave. Though current is increased in the too low load
resistance, its phase is unchanged from that in the incident wave.

The reversed-phase voltage caused by the roo-low load resistance
launches the reflected wave.

Best regards, Richard Harrison, KB5WZI