get behind vs fall back

get behind and fall back both are verbs.

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

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

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

Other hypernyms of get behind include dawdle, fall behind, lag.

get behind (verb)	fall back (verb)
to lag or linger behind	go back to bad behavior
	retreat
	hang (back) or fall (behind) in movement, progress, development, etc.
	fall backwards and down
	move back and away from
	have recourse to