It's almost always when regardless of intent, the player's actions were just too reckless and not sufficiently considerate of the safety of their opponent. Whether that applies in this particular incident, I leave to the determination of anyone watching the actual footage. Some will argue that any significant studs-to-the-opponent contact should be red carded; others argue that mitigating factors can alter that. In this case, the center ref determined that there weren't, and in the end it's his call.

Of course, because of the failure of the VAR to follow the stated protocol on reviews, the results of that review can't stand. "Fruit of the poisonous tree."