drop off vs fall behind

drop off and fall behind both are verbs.

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

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

Other synonyms of drop off include fall back, recede.

drop off (verb)	fall behind (verb)
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
retreat
remove (cargo, people, etc.) from and leave