fall back vs drop off

fall back and drop off both are verbs.

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

As verbs, fall back and drop off are synonyms defined as:

Other synonyms of fall back include fall behind, recede.

fall back (verb)	drop off (verb)
go back to bad behavior	change from a waking to a sleeping state
retreat	fall to a lower standard
hang (back) or fall (behind) in movement, progress, development, etc.	fall or diminish
fall backwards and down	retreat
move back and away from	remove (cargo, people, etc.) from and leave
have recourse to