The trigger of a gun is a small lever which you pull to fire it.

The trigger of a bomb is the device which causes it to explode.

To trigger a bomb or system means to cause it to work.

If something triggers an event or situation, it causes it to begin to happen or exist.

If something acts as a trigger for another thing such as an illness, event, or situation, the first thing causes the second thing to begin to happen or exist.