Final answer:
Hachiya's sudden shift from detachment to alarm can be attributed to unexpected plot developments or emotional revelations, as depicted in various literary examples. These dramatic turns serve to heighten the tension and advance the narrative.
Step-by-step explanation:
Hachiya's mood suddenly changes from detachment to alarm due to various narrative mechanisms that authors use to indicate a significant shift in the plot or the emotional state of a character. Examples from literature illustrate this transition, such as an unexpected revelation leading to tears (as with Kismine), sensed danger or an imminent threat (like the encounter between the heroine and the stranger in a film, or the assassins stepping into the boy's room), a dramatic scene underscored by music, or even a character's changing demeanor during a social interaction that unsettles another character (evident with Dexter's uneasiness).
In essence, a character's mood likely changes suddenly because the situation they are in has taken an unforeseen turn, or because they have received news that dramatically alters their outlook. Such shifts serve as pivotal elements in storytelling, heightening drama and moving the plot forward.