Sanctuary from predation and the safety of a home. Home is no longer home sweet home. The partnering of coral and zooxanthellae was once a powerful evolutionary achievement between them they built the reef. And now, between them, they may just let it go, dust to dust, coral reefs to their constituent parts. The Gogo reef was a different kind of reef, no symbiosis between polyp and algae, more a community of sponges and sea mosses that formed hard skeletons.
Oceans come and oceans go.
Once they cut a deal with the corals, but they didn't mean forever. Deals come apart and circumstances change. The earth gets warmer and corals get a dose of stress. There is an important lesson to be learned in the long-term view, from the coral fossil record. Corals are good predictors of mass extinction events.