drop back vs fall behind

drop back and fall behind both are verbs.

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

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

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

Other hypernyms of drop back include dawdle, fall back, lag.

drop back (verb)	fall behind (verb)
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.