Why do people suddenly look back if you look at them for a while? You possibly are familiar with the following situation, I do not know if this is a researched phenomena or not however.

You look straight at somebody for some time and suddenly, even though he or she does not consciously know that you are looking at him or her, he or she suddenly starts to look back at you.

I am wondering why and how this is happening?

How are people unconsciously recognizing other people looking at them and what makes them look back to them?
What you describe is an illusion.

(a) The human field of view is almost 180° when staring straight ahead and 270° with eyeball rotation (looking to the side without turning your head).
If you look at someone from slightly behind and to the side, they will appear to be gazing forward, and you may feel unnoticed, but in fact you are within their field of view.



(b) The peripheral field of view is highly sensitive to motion. So if you turn your head towards someone while you are within their peripheral vision, their attention will automatically focus on this (danger! predator!) stimulus and they will turn their head to you. This is a reflex.

(c) On the bus or subway, if someone sits in your field of vision, they will have noticed you when you or they came in and therefore know you are there. If you both are in a public place, they might not have seen you arrive, but they will know that someone might have. Self-conscious people feel observed, and they continually check if someone watches them, who it is, and how they react. Now if you look in the general direction of a self-conscious person, and this person looks around like they do every few seconds, this movement will cause you to look at them in reflex, giving them the impression that you have been staring at them all along. If both of you stay in the same place for some time and they continually check on you, and their movement makes you look up almost every time, they will feel like you are watching them all the time, although you are focused on something else and only react to them looking at you.

(d) Please also look at Eoin's great answer explaining illusory correlation.