The most common causes of electrical connection and wiring issues in HVAC systems include improper installation, inadequate maintenance, environmental factors, and wear and tear. Improper installation can lead to loose connections, while inadequate maintenance can cause corrosion and insulation damage. Environmental factors, such as high temperatures, humidity, and exposure to chemicals, can also contribute to wiring degradation. Wear and tear, including vibration and mechanical stress, can cause wires to fray or terminals to loosen over time.