A kite flies in the air because the profile of the kite, like the wing of an airplane, creates different fast air currents above and below the kite, which pull the kite upwards. If this current breaks off, or if the force of this current becomes so small that it can no longer act against the kite's own weight, then the laws of gravity apply: the kite will turn towards its centre of gravity and crash.
Very often a stall occurs in light inland winds when the wind drops completely in between. The kite gets no more lift and falls back. This situation is very dangerous when gusty conditions prevail, e.g. under clouds, and after a short lull a strong gust directly hits the falling kite. The kite has then already fallen deep into the wind window where it can unfold its full power. If a gust hits the kite here, the kiter may not be able to control this sudden power development. In appropriate conditions, it is therefore advisable to activate the safety directly in a stall to prevent being hit by the following gust.
Apart from stalling due to insufficient wind, there are two other possibilities which can lead to a stall, but which are based on driving and steering errors.
You risk a frontstall when you steer the kite out of the wind window, so that there is no more pressure on the holding lines. This often happens when steering the kite to the 12 o'clock position. If the kite is flown behind the kiter at this point, the current will break off and the kite will tip over and start to spin. A well trimmed kite should stand in front of the kiter at 12 o'clock without steering commands. However, if the kite tends to fly over the kiter at this point, which can result in a stall, then the trim of the kite must be corrected.
A backstall is caused when you pull the bar too hard on a kite that is very low in the wind window. This changes the flow conditions on the kite so that the suction is reversed: the kite is no longer pulled upwards but sucked downwards. It then flies backwards instead of forwards, begins to tilt backwards and steering commands are no longer accepted. The backstall is a typical steering mistake that a kiter makes at the beginning of his career. As soon as you have developed a feeling for such situations, you will immediately depower the kite strongly in an imminent backstall and thus bring the kite back into a flyable angle of attack. If you have the opportunity, you can also pull on the front lines (the lines that converge at the adjuster in the middle) to maneuver the kite faster out of the dangerous situation. Afterwards you should accelerate the kite quickly and strongly to restore the current that provides the lift.
If a kite has a permanent tendency to backstall, this indicates a feeler when trimming the kite. In this case, the backlines are probably too short and must be attached to the bar system or kite in a different way or extended a little bit.