The trigger incident is another expression for the inciting incident, or the action that begins, or "triggers," the main conflict of the narrative.
In the study of plot, especially that of short stories, there are several parts. One of these parts of the plot is the inciting incident, or as it is sometimes called, the trigger incident. This incident marks the beginning of internal or external conflict(s).
For instance, in one of O. Henry's short stories, "One Thousand Dollars," the main character Bobby Gillian, who has always been a spendthrift with the generous allowance he receives from his uncle while he is alive, finds after his uncle's death that he has inherited the sum of simply one thousand dollars. Lawyer Tolman informs Gillian that he has inherited this sum of money and further instructs Gillian,
"You are required to render to us an account of the manner of expenditure of the $1,000.00 as soon as you have disposed of it."
The receiving of this inheritance from his uncle for which he must account is the trigger incident in O. Henry's story because it sets in motion the conflicts that Gillian has about how to spend it.
Once he leaves the law offices, Gillian talks with Old Bryson at the men's club, and he asks Bryson advice that Bryson gives with his usual droll nature. But, his advice is insincere...So Gillian tries again, and he finds another dead end, and the plot develops further until he spends the one thousand dollars.