Angle of attack is not only deliberately increased by the pilot – it can be increased by external factors such as updraughts or gusts.
A pilot flying at a relatively low airspeed and high angle of attack in smooth air should have no trouble maintaining straight and level flight. However if an updraught or gust is encountered, the relative airflow may suddenly be from below the wing – increasing the angle of attack.
If the pilot were already flying close to the stall, this change in angle of attack may be enough to cause the wing to exceed the critical angle of attack and the aircraft will stall.
Updraughts, gusts and turbulence can all cause a stall in this manner but most of these external factors are not smooth and symmetrical. It is common for one wing to be more affected than the other and for the angle of attack of each wing to differ as a result.
This can lead to one wing being further stalled than the other or for just one wing to become stalled. The wing that is further stalled will have less lift, causing it to drop, and more drag, creating yaw towards the descending wing.
As the wing drops the relative airflow will arrive from further beneath it, causing another increase in angle of attack. This can worsen the stall, resulting in further roll and yaw.
If the pilot does not reduce the angle of attack, a spin may develop.