Sounds like something that happened to a guy on RC. He moved his sandbed and folks there think it was the sulfate stirred up from the anaerobic part of the bed that caused the wipeout.
Sorry to hear about the loss
Heck, I've moved 3" sandbeds a few times, too, without any loss. I wonder what we did differently to have such different results? Hmmm....
Whenever I moved a sandbed, I was usually moving the tank from one house to another. That meant all rock went into pails with tankwater, corals and fish in pails in other pails of tankwater, and finally, the sandbed in pails with just a bit of tankwater. When I got to the new location, I cleaned and rinsed the tank well. By that time, the sandbed stank bad, so I scooped the sand into the tank leaving the stinky water behind. Then I put in some of the rock, then added the old water, put in more rock and added old water and so on. I made sure I had NSW made at the new location to top up water that would be lost. PHs would be put into the tank as soon as there was enough water, usually when the tank was only half full.
I'm wondering if maybe having the sandbed in pails with some tankwater for a bit and discarding that water may have saved the day in my situations.