drop down vs fall open

drop down and fall open both are verbs.

Word	Noun	Adjective	Verb	Adverb
drop down	No	No	Yes	No
fall open	No	No	Yes	No

As verbs, fall open is a hyponym of drop down; that is, fall open is a word with a more specific, narrower meaning than drop down:

Other hyponyms of drop down include drop open, droop, flag, sag, swag, sag, sag down.

drop down (verb)	fall open (verb)
fall or descend to a lower place or level	open involuntarily