Lol.... This is funny.
The temp goes down because of the formula PV/T = PV/T. Any high school student doing Physics will tell you that.
P = Pressure (the contents of the can is under pressure)
V = Volume (the can has space)
T = Temperature (in Kelvins, which is just Celcius + 273)
The left hand side (of the equation) can be considered the conditions of the can before usage, and the right hand side (of the equation) is the conditions of the can after usage.
Anyway, the volume of the can is constant (so the contents are also constant). As you spray, the pressure drops. Therefore, for the left hand side to equal the right hand side, the temperature drops too. The more you spray, the lower the pressure drops, and the lower the temp drops. And since you sprayed non-stop for a considerable amount of time, it goes below 0C, and ice forms
And yes, I know my can contains tetraflouro ethane, and its not a real good thing to have breathing in in large quantities. I'm not sure why you'd use one to clean your keyboard, since a brush or something will do the same job. I use mine to clean dust off my heatsinks.