Being a mother is complicated because its not just a paternal culture making demands on you; its those internal demands and expectations that women have and are self-generated.
I think because mothers usually are the people who take care of us when we're little, and when we're little those mothers are omnipotent, perhaps men even more than women don't like to think about that dependency. That dependency is horror.
No matter how brilliant or accomplished they are, there is something emasculating for men in being pitted against a woman. It is even more true in creative fields already considered to be "squishy" and feminine, and it's a big problem because great women have been left off the record.
I myself have perceived women's actions as more aggressive than I would have in men because I too am walking around with my own biases. The way to fight them is to become conscious of them.