My guess would be phosphates. The only time I have ever really had problems with sps was with high phosphates. Nitrates don't seem to bother them all that much (within reason of course). When I had a phosphate spike a few years back in a smaller sps tank I was keeping, everything browned out VERY fast. But nothing actually died off, so maybe its not that.
Man this is so terrible to hear! I hope you get things back in order soon...
EDIT: I have also had a cucumber die in my tank and it did kill all the fish but never effected the corals. Perhaps I was just lucky though.