trigger vs happen

trigger is a noun but happen is not a noun.

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

trigger (noun)	happen (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

trigger (verb)	happen (verb)
release or pull the trigger on	come to pass
put in motion or move to act	come into being; become reality
	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