Has nothing to do with nature or physical damage. Molten Crash is an AM Check. If you have a) an active Blood Shield or b) the recently used Death Strike Buff from Deathstrike or Marrowrend you wont be knocked.
As nnyco has stated Blood Shield was always our AM. Iirc the "recently used DS" Buff was introduced back in WoD during HFC because it was not uncommon that Blood Shields were eaten up right before the AM check happened so you were screwed quite often.
As you can see here:
http://www.wowhead.com/spell=200723/molten-crash
Hit the check Box for Warrior or Guardian. Shield Block and Ironfur will prevent the Knockback although its nature Damage