a traitor priest burned my army!

[Norfleet]

That sounds like Phoenix Pyre. When a unit with Phoenix Pyre on dies, he explodes, and respawns elsewhere on the field. Your priest probably was simply trying to protect himself from being killed, but one side effect of the dying is that it causes an explosion. This explosion usually should harm primarily enemies, if the enemies kill you in melee combat and are therefore nearby, but if you are killed by long-ranged spells and arrows, you will explode without enemies being caught in the bLast radius. If the friendlies haven't cleared out, they will be caught in the bLast instead.