I personally think that what attracts the bullies is the look and behavior of their victim. From what I have seen at my schools, bullies are normally searching for easy victims. They want a victim that will not respond too much to their attack because if the situation becomes out of control, they want to be exit fast and without any humiliation.
The kind of behavior that attracts bullies is somebody that is shy, quiet, doesn't reply, solitary, lacking self-confidence and a lot of others. If somebody reply the first time he is attacked by the bullies, chances are that he or she will not be attacked again. People are always searching for easy things and easy victims are their prey.