Originally posted by PaunchyFool

They either died while on fire, or while they were downed an enemy Hunter threw a (hell)fire bomb or lantern causing them to burn through their heath bars which when all bars are depleted they are Burned Out and cannot be revived

When you die while burning, the fire goes out on death and has to be reignited. Of course, if you die in a burnign area, you would start burning again right away. :)