What makes a signal bounce off of a satellite dish?

An artificial satellite used for telecommunications is not a simple passive reflector. It's actually an active radio repeater; that is, it incorporates a radio receiver and transmitter ... usually several of them ... along with their antennas. In doing its job, it receives the signals aimed at it, on the frequency that it's built for, and retransmits the information at high power back down to the surface ... for example, to all those little TV dishes on most of the houses in some neighborhoods. There are thousands of "microwave radio repeaters" all over the US, usually on the ground right next to those towers you see everywhere with the 'dish' antennas mounted on them. The TV satellite does exactly the same job as the terrestrial repeater. It just happens to be working over links of more than 22,000 miles. (With no air over most of the distance, that's actually a pretty easy job for radio engineers these days.)