Overstimulation: Young Cats

Sometimes cats nip/bite for more than one reason; this can make it difficult to figure out their triggers.
Casey and Nessa are two such cats.
Being young, their ages may have played a role in nipping/biting. Perhaps they had pent-up energy due their young age and/or confinement; with this in mind, I would first engage them in play. Perhaps they had not yet learned impulse control.
All young cats need to learn to channel their energy appropriately, and the more energy a cat has, the more structured their day should be.
Casey was easily spooked by nearby activity, whereas Nessa seemed to be wound-up by nearby activity. Casey was also sensitive to touch in certain areas.
Cats like these can be easily dismissed as being aggressive. Because they're misunderstood and often challenging, they're among my favorite cats to help in the shelter.