How to explain the reflected voltage on the oscilloscope when a pulse is sent through a transmission line with a capacitor at the load?
When you send the pulse down the transmission line (t-line), initially the source of the pulse cannot know what load is at the far end so, in order to initially pass current, it uses the t-line's characteristic impedance (50 ohm, 75 ohm etc.) and produces a current proportionate with the voltage and that resistance (i.e. ohms law).

So, for a period of time you have a voltage and current "front" hurtling down the t-line and, if the terminating impedance was matching the characteristic impedance, the power associated with the two "fronts" would be totally dissipated by the load.

But your load is a capacitor and, it is receiving real power AND that power cannot be dissipated because the load is a capacitor hence, it is reflected back to the source (because that power has to travel somewhere). If the source has a driving impedance that matches the t-line's characteristic impedance, then the power is dissipated  back at the source and you get the waveform you see.

If the source has a zero ohm driving impedance then the power is once again reflected back down the t-line to the load-end and, once again, it cannot be dissipated by the capacitor and things continue back and forth.