Hallowed Ground, Benediction and Holmgang all suffer from server lag (honestly, everything does). On your screen, the enemy hits you and your HP goes into "uh oh! Time to pop hallowed ground" range. On the server side, your HP has already been there for a second(?) or so. So you pop hallowed ground while alive, but by the time HG reaches the server, the boss hit you again. And you are dead. So even though the text on the screen reads that you used Hallowed Ground (or Benediction or Holmgang) before the damage hits, the server sees it the other way.

Dead.