Of course, that's a pretty complicated question so let's see what we know about electronic system physics. Let's say we have a matched system and switch the impedance of the load to a mismatched condition. You seem to agree that the newly arrived reflected voltage/current phasors can change the slope of the load-line in the source which causes conditions to change back upstream, as you described. There is necessarily something flowing back upstream delivering information to the other stages at the speed of light in the medium. What is the nature of that electronic communications? If that communications impulse is traveling at the speed of light in the medium, the communications must necessarily be in the form of photons because electrons cannot travel that fast and there is no other carrier of electronic information besides photons that could accomplish that information transfer in a simple electronics circuit. (There may be other esoteric quantum effects that can accomplish that information transfer but I believe we can ignore such for our discussion here). Now we need to make a list of all phenomenon capable of transferring information back upstream providing feedback to a number of electronic circuits at the speed of light in the medium. There are only two directions of RF energy flow in a circuit, forward and rearward. What do we call photons that are traveling rearward? Reflected energy? Using the process of elimination, what else could it be? Seems to me that not only do the incident reflections cause a direct change in the load-line of the source, but also ripple back through the drive circuitry to cause a further indirect change in that same load-line. Ask yourself, can a load provide load information back to a source at faster than the speed of light? The lumped-circuit model says yes, but it is true? If the load cannot possibly provide load information back to the source at faster than light speed, exactly what is known to physics that can travel rearward at the speed of light? - Process of elimination.