It may be that that much of a water change was enough to stress them out.
that amounts to basically a 75% water change -- more, really, since you replaced a lot of water with more sand. So it may be that even though the water parameters are fine, the water is just different, and as such, it is intolerable to the less hardy animals, namely, your corals.
Hind-sight is 20/20, but you should have used more of your old water. I think that in general, you don't want to do more than a 30% water change. In emergency cases, 50% would work, but 75% or more is really quite drastic.
Combine that with the stress from being taken from the tank, placed in buckets (with or without a pump?) and then back into a system that may or may not have the same mineral make-up... It is a dangerous recipie.
I hope the corals pull through. I would not do another water change unless you notice an ammonia spike or trite spike.
Good luck!