Which of the following is NOT a primary cause of stalls?

High altitude is indeed not a primary cause of stalls in an aircraft. Stalls occur when the airflow over the wings is disrupted, typically due to low airspeed or the angle of attack exceeding a critical threshold. While flying at higher altitudes can result in lower air density and necessitate higher groundspeed to maintain lift, altitude by itself does not directly cause a stall.

On the other hand, insufficient airspeed can lead to a stall because the wings are not generating enough lift to sustain flight. Excessively violent maneuvers can also prompt a stall by quickly raising the angle of attack beyond the critical limit. Similarly, severe wind shear can change the lift dynamics rapidly, potentially causing a stall. Therefore, the key factor that differentiates high altitude from the other options as a stall cause is that high altitude does not inherently affect the aircraft's ability to generate sufficient lift, provided airspeed remains adequate.