The Crepitus can appear in Drassen even when it is out of ore. When this happens, you get no warning from the head miner about him having to close the mine since it's already closed. After a number of days, there would be a Crepitus attack somewhere in Drassen against your militia there (assuming you have Drassen filled with militia; I'm not sure what would happen if you don't).

I wonder if you may have triggered the Crepitus without realizing it. Talking to Bob (or the Priest or Gabby) will not trigger it. Did you find and kill Crepitus in the cave below Tixa? If you did, then that should have triggered it. Also, controlling 3 of the 4 mines (excluding Chitzena) should trigger it. Did you get the cutscene where Deidranna tells Elliot to stop feeding bodies to the creatures? If you did, then the Crepitus have been triggered.