It's the concept that women have no agency. That other people are responsible for the choices of individual women.

Check out my other post on this thread-- I show three different investigations into the reality:

Regarding occupational interests:

Men prefer working with things, women prefer working with people

But if you want it explained, well above I just said, that many (not all!) IT nerds still fullfill the stereotype in my experience.

And the stereotype is unwashed nerds, who have never been close to a women in a way they wanted, but they want to, but don't know how. So they behave weird with women around.

That makes women uncomfortable and avoiding the scene. Not long hair.