What is the cause of involuntary toe flex downward plantar?

It's a reflex.: You didn't mention which toe was plantar flexed, but, if you are referring to your big toe, it's caused by some kind of stimulus to the arch of your foot. In clinical practice, we might scrape the arch of your foot with the handle of a reflex hammer. Plantar flexion of the great toe is normal. Dorsiflexion (pointing of the toe upward) is not. But I don't know what the stimulus is in your case.