Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back" is the verb. (This is pretty clear when we construct a sentence in the present continous: "we are falling back" not "we are fallbacking").
Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back" is the verb. (This is pretty clear when we construct a sentence in the present continous: "we are falling back" not "we are fallbacking").