What is the immediate cause of stall? The direct cause of stall is unclear to me.


I heard about exceeding maximum angle of attack (around 40°) ;
I heard about reaching the stall speed in the current configuration (flaps, etc).
I heard about load factor which can lead to a stall.


Are these three parameters both direct causes of stall? By "direct", I mean for example that pitch is not a direct cause to stall, since pitch let AoA vary which causes stall.

It's unclear because there is a relation between angle of attack and speed. When we increase the angle of attack, we lower speed. But at the end, what will be the cause of stall?

If both causes are direct causes, are they independent?
- It should be possible to fly very fast (faster than stall speed) and have an angle of attack of 40°.
- It should also possible to have a very low speed (lower than stall speed) and have an angle of attack which produces lift.

What parameter will win and how are they related together?

When an aircraft is on the ground, taxiing at 10kt, the speed alone makes the plane stall, regardless of the angle of attack, am I right?

I would like to know necessary conditions to stall, and sufficient conditions to stall (all conditions being direct causes).
The immediate cause of a stall is the detachement of the airflow from the wing:



Image from NASA

This happens when your Angle of Attack is too high.



How can the AoA become too high?

--For level flight:


Given a certain velocity, you will have a certain AoA that will provide the lift needed for level flight. The lower the velocity, the higher this AoA.
Given a certain wing (configuration) you will have a certain maximum allowed AoA (in the picture is the AoA for wich you have the highest value of the plot):



Image from Wikipedia

Given these two tidbits of information, you can see what is the answer to your question:


If the velocity is too low, to fly straight you need an AoA that exceeds the capabilities of your wings (or a vectorial thrust that exceeds the weight of your veichle, but I would say that it is not quite common)
If the AoA is too high, well, is quite evident that you have stalled.


--For curved flight:

During a turn you need to use a component of your lift vector to actually turn


Image from here

As you can see the lift you need to turn is larger that the lift you need to fly level (given the same speed). And to have a higher lift you need a higher AoA.

And once again back to the AoA we are: if you want to turn too much (high load factor) you need a lot of extra lift, meaning that you have to increase your AoA. 

If you exceed your wing limits, you stall



Dynamic stall

This is not exactly my cup of tea, but reading from the already linked wikipedia article:


  Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong vortex to be shed from the leading edge of the aerofoil, and travel backwards above the wing. The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.


I would say that once again we fall in the "too big AoA" case, even if for different causes.