A coexistence that should not happen
The Cavern is deep and unlit. Species from different classes live there together — species that in the field ignore each other or fight. They do not form a pack and they do not hunt. They stay. When someone descends, they react as a single thing.
The object of the behaviour
What they surround is a chemical reagent that came out of the Breaches. The Order calls it Rupture Reagent, and it is the only substance able to break an esva's form. It cannot be synthesised, it does not drop in the field, and no raid produces it. It exists only down there.
Nobody knows why
The registry entry says it plainly: "Incompatible species in stable coexistence. Cause: unknown. Object of the behaviour: identified, not understood." The Cavern auto-scales to the maximum level in the game, so the defence always matches whoever descends. The fiction keeps its silence; the mechanics do not.