A trigger is a metal lever that when pulled discharges a gun. You shoot the gun by pulling the trigger. Trigger can also mean the start of a process––violent protests might trigger a revolution.
Certain scents, places, or old love songs can be said to trigger, or activate, memories. If you get weepy when you hear the national anthem, that may be because it triggers thoughts of family, home, baseball and the Fourth of July. For someone in frail health, a seemingly minor problem like getting a common cold, might trigger a series of physical responses that end up landing them in the hospital.