How did the army of the dead get undead?

Answer 1

They were undead because they had gone back on their oath and as such were doomed to a living death. Aragorn with the sword that was broken, called them forth and allowed them the opportunity to fulfill their oaths and be free of their cursed lives. The Army of the Dead, at the time of their mortal existence, were living in the land of Erech. These people of Erech had sworn an oath to Isildur to serve him if he needed help. When Sauron attacked Gondor, Isildur called upon them to fight against Sauron and fulfill their oath. But they refused, because they had worshiped Sauron in the dark days. Then Isildur cursed them, that if he returned victorious, they would be as phantoms, always to walk Middle-earth until they fulfilled their oath.