Modern young women often possess what is commonly called a bitch shield. This is a type of behavior that she engages when she wants to drive a person away, and do so quickly. Often this other person is a man who might be interested in her. That fact that he usually quickly leaves shows that the shield had its intended effect of “protecting” her. And, of course, her behavior is usually some combination of nasty, denigrating, sarcastic, etc. which is where the bitch part of it comes from.
From what I can tell, this behavior’s prevalence is a modern occurrence. Of course, women throughout the ages have been bitchy, but ladies were normally taught not to be this way. And if they did engage in bitchiness, it was supposed to be when they were at the end of their rope. In modern times, bitchiness is often engaged quickly and strongly. And it would seem that it starts with girls in their early years.
Digging deeper, the question arises, why do they do it? Is the sole purpose to dispatch unwanted attention? Or is there more? Perhaps there is an ego component. By easily putting a guy down, they show how valuable they are. Perhaps they just enjoy the drama of it. For some reason, women apparently like drama. Perhaps it is a show for their nearby friends, or something that she can brag about on Facebook. Maybe commenters can suggest other reasons.
There does not seem to be any good reason. The most succinct assessment is that women who do this lack grace.