Women in general are more selective of mates than men are, so women are in more competition with other women for high-value men.

Women are also largely valued for things that cannot be acquired (i.e. youth, beauty, femininity), meaning that there will always be a younger, more beautiful woman to compete with for a mate’s attention. And when that happens it’s not as though she can appeal to her love interest more by becoming more successful, attaining higher status, or becoming a respected group leader. She only has a limited time to attract a man she deems fit to procreate with.

This then is the root of female group animosity: narrow-window competition.

(And before anyone gets their panties in a twist, this theory is coming from a woman).