WordNet (r) 3.0 (2006):

fall back
    v 1: fall backwards and down
    2: hang (back) or fall (behind) in movement, progress,
       development, etc. [syn: lag, dawdle, fall back, fall
       behind]
    3: move back and away from; "The enemy fell back"
    4: retreat [syn: fall back, lose, drop off, fall behind,
       recede] [ant: advance, gain, gain ground, get
       ahead, make headway, pull ahead, win]
    5: have recourse to; "The government resorted to rationing meat"
       [syn: fall back, resort, recur]
    6: go back to bad behavior; "Those who recidivate are often
       minor criminals" [syn: relapse, lapse, recidivate,
       regress, retrogress, fall back]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "fall back":
   avoid, back, back down, back out, back up, backslide,
   beat a retreat, blench, blink, come after, come last, cringe,
   degenerate, deteriorate, disengage, disimprove, dodge, draw back,
   draw off, duck, evade, fade, fall, fall again into, fall astern,
   fall behind, fall from grace, flinch, follow, get worse, give back,
   give ground, give place, go back, go backwards, grow worse,
   hang back, have a relapse, jib, lag behind, lapse, lapse back,
   let down, move back, pull back, pull out, quail, recidivate,
   recoil, recur to, reel back, regress, relapse, retire, retract,
   retreat, retrocede, retrograde, retrogress, return to, revert,
   revert to, run back, sheer off, shrink, shrink back, shy, sicken,
   sidestep, sink back, slacken, slide back, slip back, stand back,
   start aside, start back, swerve, trail, trail behind, turn aside,
   weasel, weasel out, wince, withdraw, worsen, yield again to



The Free On-line Dictionary of Computing (30 December 2018):

fall back

    A feature of a modem protocol where two
   modems which experience data corruption, e.g. due to line
   noise, can renegotiate to use a lower speed connection,
   possibly applying fall forward if the channel improves.

   (2004-07-30)