The Trait Orc Armor will activate when the Orc takes damage to Life or Armor, including Spells that damage all armor.
The Trait Orc Armor says to "Gain 5 Armor when I take damage." If you notice an enemy or one of your own Troops activating this Trait more than it seems like it should, this is due to the way the Spells work. If you cast a Spell against a Troop with Orc Armor and you remove or reduce the Armor or do damage to Life this counts as 'taking damage' and therefore the Trait activates.
Will this be changed?
The Gems of War development team is currently overhauling all the text and Spell descriptions in the game to help make situations like this clear. As this is a massive task due to a large amount of text in the game it will take some time.