Trahearne didn't know the house in question or the wards or exactly what was happening other than it was bad. Bad enough that he doubted Cassian had much time to do anything. Tannusen wouldn't give him much time, he figured.

"Should anything of the sort happen again, you will have a better idea what to do then. Right now, you'll do yourself no good by finding faults in your actions in a desperate situation. Best we think of what we will do when this blows over. Please, sit down." He was going to keep asking that until Cassian actually sat down. He wasn't moving away from Cassian until then, he couldn't risk him bolting from the door and ending up hit by the fear spell in those marks.