An AC unit can freeze up due to several reasons. Common causes include low refrigerant levels, which can cause the evaporator coil to get too cold, and restricted airflow due to dirty air filters, blocked vents, or malfunctioning fans. Additionally, thermostat issues or a clogged condensate drain can contribute to freezing. Regular maintenance, such as cleaning or replacing filters, ensuring proper airflow, and checking refrigerant levels, can help prevent this issue and keep your AC running efficiently.