fall back vs get behind

fall back and get behind both are verbs.

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

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

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

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

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