A refrigerator that freezes can be caused by a number of things, from circuit boards to a temperature control thermostat. The thermostat is the part of the refrigerator that passes power to the compressor and fans.
A failing thermistor is another possible cause, as this is the part that monitors and senses changes in the air temperature inside the refrigerator. It is also possible for the damper, a small door that opens and closes to help regulate airflow, to be stuck in the open position. Although less common, control boards can also fail, and this may result in freezing. A qualified appliance technician can generally diagnose and repair any of these problems.