I knew SO many girls like this in highschool.  They'd get these guys to go after them, then go out with someone else out of the blue, leaving the guy heart broken.  Then, later, after the guy is in a good relationship, the girl happens to break up with her boyfriend and runs back to the guy because he was her "backup" and she thinks she has control over him still.  She doesn't really like him, she's just using him because she doesn't want to be alone.