At this point they might consider MC unlucky for walking into these disasters - unless party wipes and irregular monsters are actually this common. You would expect an official anti-stampede team would be prepared for anything. Once again it will be hard to explain to the investigator whatever MC is about to pull off.
Leader guy failed to listen to his own advice and didn't retreat into the caves where the monster would have a hard time making sharp maneuvers while the party would find many hiding spots and attack angles.