The answer to 'why is my ductwork freezing up' is that it is likely due to a problem with the airflow in your HVAC system. Common causes include a dirty air filter, a malfunctioning blower fan, or a refrigerant leak. When the airflow is restricted, the temperature in the ducts can drop below freezing, causing ice to build up.
If you notice ice forming on the ductwork, the first thing to check is the air filter. A dirty or clogged filter can significantly reduce airflow. Replace the filter and see if that resolves the issue. If the problem persists, it may be due to a more serious problem like a faulty blower fan or a refrigerant leak, which will require professional repair.