What is the immediate cause of stall?
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.