Refrigerant leaks in HVAC systems can be caused by a variety of factors, including corrosion, vibration, improper installation, wear and tear, and manufacturing defects. Corrosion is a common cause of leaks, particularly in systems with copper or aluminum components, while vibration can cause connections to loosen over time. Improper installation, such as inadequate tightening of fittings or incorrect brazing techniques, can also lead to leaks. Regular maintenance and inspections can help identify and address these issues before they become major problems.