Question About Knockdowns

During Murray-Martinez, Kellerman stated that a second knockdown should've been called, because although Martinez fell due to being hit behind the head, he moved such that he was struck there. People on various forums used similar reasoning to justify a knockdown ruling, which got me wondering: are referees obligated, by written rules or conventions stemming from past cases, to call knockdowns in these situations? If so, and to those who think this obligation should exist, how is the following scenario handled: A crouching fighter suddenly stands erect and consequently their opponent, who was aiming for and would've hit their crouched body in a scoring area, lands well below the belt, dropping the fighter and keeping them down for over a minute. Should the hit fighter be ruled down and lose by KO if they are dropped and cannot get up within a ten count?