Why did supernova 1987A's “String of 'Cosmic Pearls'” appear to be so lumpy 20 years later? Is it still?

NASA's A String of 'Cosmic Pearls' surrounds an exploding star is beautiful and the page says:

This image shows the entire region around the supernova. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring's inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded.

Did the ring break up into "pearls" or lumps due to some kind of instability? Why so lumpy? The image was taken 20 years after the supernova was first observed, does the ring still look like this?

If the density of a clump is sufficiently high, radiative cooling will cause the clump to collapse to an even higher density. The result will be an optically emitting hotspot (Pun et al. 2002). If instead, the density of a clump falls below the threshold density for radiative cooling, the gas behind the transmitted shock will radiate X-rays and very little optical radiation. This threshold density is a sensitive function of the velocity, $${{V}_{{\rm clump}}}$$, of the shock entering the clump. Thus, at any given time, the requirement for radiative cooling acts as a high contrast filter, limiting optical radiation to only the denser clumps.