I think that maybe your lighting schedule is too short. Also, what bulb combination are you using?
I don't know if this is scientific fact but in my experience sudden death in SPS is usually a the result of longer series of multiple events. Low light over a long period of time, combined with a minor alk swing or other issue and suddenly you have RTN. In reality that coral may have been struggling for a long period and an event tipped it over the edge.
My cprals weren't doing well for a while and I was losing a few. I realised that I was only running my lights at full power 6 hrs a day. As soon as I ramped that up I started noticing improved health, colour, polyp extension. I have a 4 bulb ATI hybrid (4 LED clusters) I run it full power for 12 hours a day plus ramp up and ramp down time.