What is the explanation of this airfoil phenomenon? Why the air from below the leading edge's tip deflected up?
The short answer is: because not all of the incoming air can fit under the airfoil, so some of it gets pushed up and above the leading edge. This air could also fit under the airfoil by other means like becoming denser (compressing), but that requires more energy, so it mostly flows around.

In your image, the effect is exaggerated by the closeness of the ground, which places the entire airfoil in ground effect. In normal flight conditions, without the ground blocking it, more air would be able to flow underneath the airfoil.



The longer answer has to do with the speed of sound, which is the speed at which pressure waves propagate through a fluid. If the free stream is subsonic, pressure waves generated by an object can flow upstream and alter the flow ahead of any disturbance.

This is exactly what is happening here: the airfoil is affecting the pressure field, specifically by creating an area of high pressure underneath. This affects the incoming flow, deflecting it upwards ahead of the airfoil.

If this experiment were performed in supersonic conditions, you would not see any disturbance in the incoming airflow until it hit the bow shock wave of the object, after which it would become locally subsonic and be abruptly deflected to flow around the airfoil.