There's an initial flash of light from the supernova explosion causing the ring to glow. Debris hurls into space, the fastest moving at 1/10 the speed of light. The supernova's shockwave causes the ring to glow again. The closer the pieces of the ring are to the shockwave, the sooner they light up. Eventually, the whole ring lights up.
T. Goertel (STScI)
Publication: February 10, 1998