How did Eumaeus become a swineherd?

Eumaeus the swineherd explains to Odysseus, when he visits him in disguise, that servants who have "young lords" as their masters tend to live unhappy lives, because these young men do not know how to properly look after their servants. Eumaeus explains that the man for whom he worked most of his life, and from whom he would have expected a house, land, a wife, and other benefits as gratitude for a lifetime's work, is not around to provide those things for him. His master went to the war instead, so he was not able to stay, grow old, and provide for Eumaeus, his servant, as would have been expected. The swineherd's master went to Ilius to fight the Trojans for Agamemnon, and as a result, Eumaeus condemns Helen and her tribe for having taken his master away from him.
Today, Eumaeus is in charge of the pigs but must now pick out the best ones he can and send them to the noblemen. His master was, of course, Odysseus, but Eumaeus does not recognize him and does not believe that he will ever return.