If someone has a habit of bending his neck down all the time then what can we call this? Is there any general name for this type of habit?
In general, it may be called a mannerism.
- a habitual or characteristic manner, mode, or way of doing something; distinctive quality or style, as in behavior or speech: "He has an annoying mannerism of tapping his fingers while he talks."
If there is a medical jargon/terminology for this, I am honestly not aware; but in general, such may be called a mannerism.