return vs fall back

return is a noun but fall back is not a noun.

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

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

Other hyponyms of return include lapse, recidivate, regress, relapse, retrogress, resile, go back, recover, recuperate.

return (noun)	fall back (noun)
the act of going back to a prior location
a coming to or returning home
getting something back again
a tennis stroke that sends the ball back to the other player
(American football) the act of running back the ball after a kickoff or punt or interception or fumble
a reciprocal group action
the act of someone appearing again
the key on electric typewriters or computer keyboards that causes a carriage return and a line feed
document giving the tax collector information about the taxpayer's tax liability
a quick reply to a question or remark (especially a witty or critical one)
happening again (especially at regular intervals)
the occurrence of a change in direction back in the opposite direction
the income or profit arising from such transactions as the sale of land or other property

return (verb)	fall back (verb)
go back to a previous state	go back to bad behavior
be restored	retreat
go back to something earlier	hang (back) or fall (behind) in movement, progress, development, etc.
answer back	fall backwards and down
submit (a report, etc.) to someone in authority	move back and away from
pass down	have recourse to
make a return
give or supply
go or come back to place, condition, or activity where one has been before
return to a previous position; in mathematics
bring back to the point of departure
pay back
return in kind
give back
be inherited by
elect again