An easy way to understand what is going on with signal reflections is to consider a very long wire -- say, to the moon. When you put a voltage on the wire current starts flowing, but that current has nothing to do with what is on the other end of the wire, because it is far away and there is this thing called the speed of light. So the current has to do not with the load but with characteristics of the wire. Eventually the voltage/current will get to the other end of the wire -- the reflection is what happens when the voltage/current in the wire does not match what is required by the load connected to the end of the wire. The problem, especially at high frequencies, is that this doesn't require a wire to the moon to see, it can happen in inches. Try debugging a 400G Ethernet link composed of 16 traces about 12 inches long on a PC board -- can be very hairy.