I think that as long as the dragon doesn't walk around, the game is thinking that since one mine exploded and there is no enemy movement where they are placed, then the enemy is standing where the first mine was triggered and the rest will be idle until there's movement.
I'm pretty sure this is related to the change that was implemented a while back, around the time Blood for Blood and Thrassians were new. Back then a guy posted a parse from the pts where he used mines as a spammable cause they all exploded on the dummy at the same time. This was changed with the first pts update after that.
