Copper Doors and Trapdoors have a blast resistance of 3, same as wood, rather than 6, like every other Copper block (including blocks that might be more "fragile", like Copper Bulbs and Lightning Rods.) This is unlike every other door or similar building block in the game, where the blast resistance is equal to that of the material it's made from.

This means that Copper Doors/Trapdoors can be destroyed by Fireballs. This can be demonstrated in creative by summon a fireball and punching it into a copper door or trapdoor.