trigger vs come about

trigger is a noun but come about is not a noun.

Word	Noun	Adjective	Verb	Adverb
trigger	Yes	No	Yes	No
come about	No	No	Yes	No

trigger (noun)	come about (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)	come about (verb)
release or pull the trigger on	come to pass
put in motion or move to act