I'm sure you were very careful removing the rocks, but if the rocks were removed first, in order to remove the fish more easily, one of the rocks could have leaned or fallen onto him, inadvertently causing pressure on him or his swim bladder. I know wrasses I have had over the years always head to a hiding spot beneath rocks, right on the sand or bottom, when stressed. I could be completely wrong here, but if he was fine and healthy before the transfer, I would think a disease or other ailment would be out of the question, leaving a few possibilities as to why he died. They are so delicate, it doesn't take much pressure to cause an internal injury. The other fish are all doing ok? Again, really sorry for your loss, I hate losing fish, especially in situations like this, where you are not 100% certain of what caused it. Maybe the reefsquad will have a better diagnosis than me, without seeing him or being there, I am really just brainstorming and trying to help best I can.