fall open vs drop down

fall open and drop down both are verbs.

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

As verbs, drop down is a hypernym of fall open; that is, drop down is a word with a broader meaning than fall open:

Other hypernyms of fall open include drop, sink.

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