My guess is one of two possibilities...
1) any chance that you have trace remains of copper in your tank? Was this a used tank, or have you ever used copper? Starfish have a zero tolerance to copper.
2) I'm more inclined to lean towards this....some inverts are extremely sensitive to variations in water, such as temp, ph, etc., and require much longer/slower acclimation times. I learned this with shrimp after going thru 4 or 5. Some would die within an hour, some a week later. But after I tripled my acclimation time and slowly adjusted them to the new water, no problems. Some people never have this issue and can just bring something home, float the bag for 10 mins., and drop them straight in. I have never been this lucky.