Redirected aggression
An animal worked up by something it cannot reach turns on whoever is nearest — the classic cat-at-the-window incident.
A cat watching a strange cat through the glass is fully aroused with nowhere to put it. The nearest housemate, or the hand that reaches down to comfort it, receives what the window would have received. It has nothing to do with the target and everything to do with the arousal, which is why the victim is so often the animal with the best relationship.
Recovery can take hours, and reintroducing two cats after an incident is usually slower than owners expect — separate rooms, then scent, then distance, then contact. Blocking the view of the trigger is the part that stops it happening again.