Fall back and Go back on

Fall back verb - To move back or away (as from something difficult, dangerous, or disagreeable).

Go back on and fall back are semantically related In some cases you can use "Go back on" instead a verb "Fall back".

Go back on verb - Fail to fulfill a promise or obligation.

Fall back and go back on are semantically related in renege on topic. Sometimes you can use "Fall back" instead a verb "Go back on".