Nothing you have told us here would explain this event IMO. You alkalinity is higher than I like but if its been stable than I dont think it to be the cause.
Sometimes we never know the cause (although Im sure it's something/or a combination of things). All I can suggest is to do lots of water changes for a period and do not let the dieing SPS linger in the aquarium too long. Many people believe that it can be bacterial/virus and can go from colony to colony. I think its more likely that they are all in the same environment and react similarly to the negative stimulus, but you never know. And the dead SPS tissue can't be good for water quality.
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