The answer to 'why is my ductwork freezing up' is that there is likely an issue with the airflow or refrigerant level in your HVAC system. This can cause the coils or ductwork to freeze, restricting airflow and leading to further problems.
Some common causes include a dirty air filter, low refrigerant, a malfunctioning blower motor, or a problem with the thermostat or control board. Symptoms may include reduced airflow, ice buildup, and poor heating/cooling performance. Solutions can range from replacing the air filter to repairing refrigerant leaks or other component failures. In many cases, it's best to call a professional HVAC technician to diagnose and fix the underlying issue.