When a Biter or Spitter dies it tries to reincarnate into the true perpetual enemy, a tree. Occasionally the very angry ones fail and burst into flames.
The tree type (living and dead) will be biome appropriate for where the biter dies. Only base game tiles and tree types are supported as the mod uses the base game's map generation noise layer data. When the death is on a player placed ground type (floor) the underlying tile type is used. There is a configurable (1% default) chance of a biter death causing a tree fire at that spot.