happen vs trigger

happen is not a noun while trigger is a noun.

Word	Noun	Adjective	Verb	Adverb
happen	No	No	Yes	No
trigger	Yes	No	Yes	No

happen (noun)	trigger (noun)
	an act that sets in motion some course of events
	lever that activates the firing mechanism of a gun
	a device that activates or releases or causes something to happen

happen (verb)	trigger (verb)
come to pass	release or pull the trigger on
come into being; become reality	put in motion or move to act
happen, occur, or be the case in the course of events or by chance
come upon, as if by accident; meet with
chance to be or do something, without intention or causation