The AI played a Fate Egg. (I'm not sure if it was) In the same or the next turn (but I think it was in the next turn) it used a SoR and the egg hatched. It gave the AI two Shriekers. The first (which took the slot of the egg) immediately burrowed itself. The other one did not, although there was enough quanta (10) left.
I assume it's an AI only bug and thus complicated to recreate.