fall back vs drop back

fall back and drop back both are verbs.

Word	Noun	Adjective	Verb	Adverb
fall back	No	No	Yes	No
drop back	No	No	Yes	No

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

fall back: hang (back) or fall (behind) in movement, progress, development, etc.
drop back: to lag or linger behind

Other hyponyms of fall back include drag, drop behind, get behind, hang back, trail.

fall back (verb)	drop back (verb)
go back to bad behavior	take position in the rear, as in a military formation or in the line of scrimmage in football
retreat	to lag or linger behind
hang (back) or fall (behind) in movement, progress, development, etc.
fall backwards and down
move back and away from
have recourse to