It's strange, isn't it. You stand in the middle of a library and go "aaaaagghhhh" and everyone just stares at you. But you do the same thing on an aeroplane, and everyone joins in.

The joke hinges on social context and contagious behavior. In a library—coded as quiet, orderly, and rule-bound—an inexplicable scream is treated as deviant and draws disapproving attention. On an aeroplane, the same scream is reinterpreted as a plausible signal of danger, so it spreads rapidly as collective panic. Cooper’s line plays with how people take cues from their surroundings and from one another, and how “appropriate” reactions are often less about the act itself than the setting in which it occurs. The humor comes from the sudden shift from individual embarrassment to mass participation.