Refrigerant line restrictions can be caused by internal corrosion, external damage, dirty components, and other factors. Internal corrosion can occur due to moisture ingress, acid formation, or reaction with refrigerant oils. External damage can result from physical impact, vibration, or environmental factors. Dirty components, such as clogged filters or dirty condenser coils, can also restrict refrigerant flow. Additionally, improper installation, manufacturing defects, or aging of components can contribute to refrigerant line restrictions.