it can be million reason why acros die. sometimes one can just RTN for no reason. New Corals will RTN from stress, temp swing, dip too long, Not dipping, too much light, too little light etc....
few questions.
tank size?
Alkalinity
calcium
magnesium
Ammonia?
nitrate?
phosphate?
what are you dosing?
what lights?
If you have a small tank? it can be parameter swings that cause them to die. I wish there is a simple answer.
Best advice do large water change for next few weeks. Stop all chemical dosing. worse come to worse. stop and reboot.