This is because the CO2 fire extinguisher is held as a liquid under pressure within the cylinder, and when it is discharged it expands rapidly (450 to 1!), cooling down the material it contacts. As such, during the discharge of a 2kg CO2 fire extinguisher, the temperature of a traditional horn drops by 40oC, giving the user a risk of frost burn if the horn is accidentally touched. The icing is water vapour in the air freezing.
A check of the old MoF part 1 1943 & 1963 (More detailed science in these) states temperature changes like this are due to Boyles Law- the inverse of the above is when increasing the pressure of gas in a cylinder you get heating up - when decant filling the old Sabre 'inhaler' portable oxygen cylinders from a larger 'F' size I had to keep one hand on the valve spanner of the donor and one on the recipient as if I filled too quickly the cylinder would heat up rapidly, (hich with oxygen isn't wise!) and I'd have to tighten off the donor valve & slow the filling rate.
Frost free horns work in two ways: the original Zahan is double skin - a horn within a horn - so the inner may frost but not the outer (however in practice neither frost due to air flow between the skins); and the other makes are simply thicker single skins.
The Zahan patent explains the detail of how it works, including how it has allowed the 2kg CO2 to gain a 55B rating instead of 34B -
http://www.freepatentsonline.com/EP1341584.html
