Why would frost form on the outside of a fire extinguisher?

When looking at this use the equation PV = nRT. From this you can see that temperature and pressure are proportional to one another. So if your temperature increases for the same volume and number of moles your pressure must also increase. When you use a fire extinguisher you rapidly decrease the pressure inside the container which causes the temperature to decrease. The frost you see is water vapor from the air freezing when it comes into contact with the cold outside of the extinguisher.