When an enemy unit or worm dies it tries to reincarnate back in to nature, be it a tree, rock, cliff or water. Occasionally the angry biters can burst into a flaming tree.
Biters as cliff would run the risk of trapping you in a radius where you lack resources before you can ever get to cliff explosives. It would need to have a check to make sure it can never produce a continuous, impassable terrain.
