Athanasius points to the biblical teaching of creation from nothing to prove that creation is in its nature changeable.  It’s not simply that something comes from nothing is fragile, unstable, dependent; it also seems that creatures have a changeable nature because their origin is change:

Scripture “teaches that [the Son] changes everything else, and is Himself not changed, in saying, ‘You are the same, and Your years shall not fail.’ And with reason; for things originate, being from nothing , and not being before their origination, because, in truth, they come to be after not being, have a nature which is changeable.”

He’s assuming that origin determines nature (“flesh from flesh, Spirit from Spirit”), and if creation begins, it originates in and as a moment of change, thus making mutability its essence.