Whilst I know it's ridiculous and not the way it's intended or even remotely in the spirit of the game, it was brought up somewhere and RAW "In addition, any enemy model that makes a successful charge against Gor-Rok and ends the movement phase in base contact with him must test"
If you fail a dangerous terrain test the model is removed, it's not a casualty removed from the back rank of the unit, and therefore another unit is forced to "move" into base contact with Gor-Rok triggering the shield of aeons again.