I'd frag as many pieces off as possible, leave them unmounted and scatter them around the tank. Then get rid of what's left of the colony, get it out of your tank.
Make sure you frag into good, strong living tissue. Anything else and you might as well write off the frag too. If the frags don't RTN on you in the next week or so, I'd mount some of them at that point.
A single 1" frag that survives is better than nothing. And it sounds like you are having a RTN episode, and nothing is what you will soon be left with if you don't frag it.
As to what causes it, who knows. Probably a combination of all of the factors you mentioned resulting from the move....stressed the colony, and allowed some type of infection-type "bug" to attack the colony. (total speculation on my part, nobody's been able to really nail down the cause of this......it even happens occasionally on stable tanks/colonies). good luck!