[Question]: RE - Fire Trap and Cast When Damage Taken

Using the Fire Trap skill makes you place a trap in the world, that when triggered, casts a spell. At no point do you cast a spell. You aren't casting anything, so this can't be triggered.

Ethereal Knives is you casting a spell that summons magic knives out of the air and flings them away from you. This is a spell you cast which can therefore be triggered.