Some of this information suggests that one of the overlying beds, that may not have dropped earlier, suddenly broke and dropped down onto the underlying rock pile. That hanging up, and then failing to drop over a large area is what might happen if there were a thick sandstone layer in the roof, but if the cavity transferred upwards I would have thought that it might have been detected in the drilling activity, since intersection with a cavity would lead to lost circulation in the drilling fluid. (Though I don't know what type of drill they are using).