It’s not mysterious at all, however. It’s just science.
Electric currents and magnetic fields are closely related. If you have one, you often have (or can create) the other. The braking effect that happens to the magnet at it travels down the tube is due to an eddy current. That’s an electrical current that forms in the copper in response to the magnetic field that generated by the moving magnet.
The eddy current then creates a magnetic field of its own, causing the two magnetic fields to overlap, thus slowing the magnet as it falls.