chance event vs happening

chance event and happening both are nouns.

Word	Noun	Adjective	Verb	Adverb
chance event	Yes	No	No	No
happening	Yes	No	No	No

As nouns, happening is a hypernym of chance event; that is, happening is a word with a broader meaning than chance event:

chance event: anything that happens suddenly or by chance without an apparent cause
happening: an event that happens

Other hypernyms of chance event include natural event, occurrence, occurrent.

chance event (noun)	happening (noun)
anything that happens suddenly or by chance without an apparent cause	an event that happens