Why would a woman keep staring at another woman?

Either she is interested or jealous.

People look at each other all the time. It usually means absolutely nothing and has no particular reason to it. Maybe she liked her shoes. Maybe she admired her walk and wondered if she could incorporate that attitude into her own. Maybe she thought the other gal the hottest thing since sliced bread. Maybe the gal reminded her of her mother. Who knows?

Jealousy and envy would be two reasons. Or maybe she just liked the other woman's style.

Maybe she was checking her out in a non lesbo way