retire vs fall back

retire and fall back both are verbs.

Word	Noun	Adjective	Verb	Adverb
retire	No	No	Yes	No
fall back	No	No	Yes	No

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

Other hyponyms of retire include retreat, retrograde, back down, back off, back up.

retire (verb)	fall back (verb)
prepare for sleep	go back to bad behavior
cause to get out	retreat
cause to be out on a fielding play	hang (back) or fall (behind) in movement, progress, development, etc.
lose interest	fall backwards and down
pull back or move away or backward	move back and away from
dispose of (something no longer useful or needed)	have recourse to
break from a meeting or gathering
go into retirement; stop performing one's work or withdraw from one's position
withdraw from active participation
make (someone) retire
withdraw from circulation or from the market, as of bills, shares, and bonds