Why are women so hostile towards each other?

So we had this cute new girl join our firm this past month. I and the other guys have noticed a lot of hostility towards her by the other girls in the office. When we go out to lunch, she eats with us, 3 or 4 guys, instead of the other girls. She's also been rudely cut off while speaking by the other women during meetings. They go out of their way to make her feel unwelcome. I don't get why women are so threatened by a pretty, smart girl in the workplace of all places ? I mean its not like we're banging each other that the girls should be defending their turf or something. I don't get it