Why is my roommates cat attacking him?

I got a new Kitten, a little over a month ago. We kept them separated for a couple weeks with very low amounts of interaction. His cat wasn t a big fan but didn t seem to nearly as angry as she did with dogs. Then we slowly let the kitten roam around a bit... Making sure to do it room by room. Everything seemed fine. The other cat wasn t being aggressive, really, unless the kitten started messing with her. They even played with each other a few times. However, a couple days ago (start of the 6th week with the new Kitten), my roommate was opening a package.... And his cat (not the kitten) pounces on him and starts biting and scratching him. She doesn t hiss at me, and doesn t even hiss at the kitten....but straight up attacks my roommate at seemingly random. Any ideas? My roommate thinks it s redirected aggression due to the kitten, but I told him that he might want to call a vet.