I remember that around 10-20 years ago, in Peru, there was this case of an near entire village beign found dead. From what very little surivors were left, they said that as they open their door to see what this panic was coming from, they would drop dead in the door way. It was later found that due to the geological conditions in the area, Co2 sat at the bottle of a lake bed until a small tremor caused some Co2 to break the surface. The air density around caused the gas to get pulled to the surface, expand and start to pocket under the lighter o2 and other gases in the air. I would no be suprised if the metorite hit a pocket of Co2 or some other gas caught in the earths crust.