I may be way off, but I think she didn't like black people.
My freshman roommate was black, and I was walking from the Castilian to Jester with his dad when the woman pulled the knife on me. The other time she moved aggressively towards me, I was with my roommate.
Another time when I was going to or from the Co-op, I was with several black students, and I had warned them about her. As we were getting close to the store, the woman doggedly got on the heels of a girl, and followed her to the door, and then abruptly turned around, instead of entering the store. It scared the girl pretty badly.
After that, I stood outside and watched her for a couple of minutes, and I saw her walk past, then double back on two or three people...all black.
I know it sounds weird, but that's what happened.