"device by means of which a catch or spring is released and a mechanism set in action," 1650s, earlier tricker (1620s), from Dutch trekker "trigger," from trekken "to pull" (see trek). Tricker was the usual form in English until c. 1750. Trigger-happy "ready to shoot (or otherwise react violently) on the slightest provocation" is attested from 1942.
"cause (something) to happen," 1930, an image from trigger (n.). In recent use especially psychological, "to cause an intense and usually negative emotional reaction (in a person or animal)," by 1986. Related: Triggered; triggering.