Occasionally when I enter combat with a Raider after a few seconds they will verbally express their fear of the situation (sometimes saying something along the lines of "Screw this, keep your stuff!"), turn around, and begin running away.
As I was still interested in the fight I followed them and can only remember instances of them running a few dozen feet before cowering behind an object and then standing back up and shooting me again since I was close. This sort of retreat is rare and typically I wipe out Raiders with ease, so why do they occasionally run from me?