I saw this drop from the Warrior yesterday as well. I had never seen one of his masks drop other than when I killed him in the story. I still haven't beaten him as a Mecromancer so I was happy to take it.
Occasionally, the Warrior will drop an orange version of Handsome Jack's mask. The only difference is that the one from the Warrior is for a specific class, whereas the one from Jack I believe only works on the character you killed him with. Otherwise, there's no difference but it is a nice way to add one to the Nothing Rhymes with Orange challenge. --- 2007 - 2008 NHL Saturday Picks Champion