What is the exact cause of eye blinking?

Reflex. There is an internal rhythm which signals blinking spontaneously. It is modified by the osmolarity of the tears and the exposure of the receptors on the cornea so that there is stimulus to blink more often if these are activated. The blink spreads the tears over the living tissue on the front of the eye to keep it healthy and optically clear.
A reflex. Blinking is an involuntary neural reflex that protects the ocular surface from drying and irritants, or obnoxious stimuli like bright light or loud noises. This neural circuit involves a communication between the ocular surface neurons (sensory); the neurons that go to the muscle of blinking (motor). You can also voluntarily blink by over coming the involuntary mechanisms. The latter is cerebral.