means A trigger is the small lever you squeeze to fire a gun, or by extension anything that sets off a sudden reaction or chain of events.

from From Dutch 'trekker,' meaning a puller or pulling device, from 'trekken' — to pull or draw. It entered English in the 17th century, originally spelled 'tricker,' before settling into the smoother 'trigger.' The metaphor stays faithful to its root: a trigger is forever something you pull. The psychological sense — a stimulus that sets off a reaction — is a much later figurative bloom on that same mechanical stem.