How does low freon cause the compressor suction line to freeze up
Chlorodifluoromethane, also known as R22 refrigerant has a boiling point of -40&deg;F at 0 psi.  As you increase the pressure of the refrigerant, the boiling point also increases. At 68.5 psi, the boiling point of R22 is 40&deg;F.  

In a normally functioning system, the refrigerant is sent into the evaporator at about 55-65 psi. Which means that the boiling point will be above the freezing temperature of water. The refrigerant absorbs heat from the air passing through the evaporator, boils, and flows on down the line at a temperature above the freezing point of water.  The refrigerant will likely be below the dew point, however, so there will be condensation on the line.

If the pressure in the system drops, the R22 might be entering the evaporator a bit lower than the typical pressure. In this case, the boiling point of the R22 will be below the freezing temperature of water. As the warm moist air moves over the evaporator, the moisture condenses and freezes on the coils. Ice will start to form at the beginning of the evaporator coils, and slowly creep along its length. 

The ice will act as an insulator, so the refrigerant in the line will not be able to absorb the heat it needs to boil. This causes the refrigerant to boil off further along in the evaporator, which forms more ice further down the line.  This process continues all the way along the line, until the entire evaporator and suction line are covered in ice. 

Once the refrigerant levels get too low, there's not enough refrigerant in the system to freeze the line. So eventually if the system has a leak, this behavior will stop and you'll simply get no cooling at all from the system.



NOTE: I could be completely wrong here. This is based on my limited knowledge of air conditioning systems. I am not an expert in thermodynamics, fluid dynamics, chemistry, or HVAC.