Very sorry but the answer is probably that the bacteria may have been weak I.e. Starved of oxygen during the move (it begins to die within 30 mins of no flow) and then no bio load with with another shock when the rays turn up.
If you re_filled without dechlor then the bacteria could be further destroyed and the fish could have been weak due to high nitrate or other params before moving.
The answer is experience coupled with major water testing be fire and after the move.......
I am truly sorry for the loss.