Why did Telemachus return home to Ithaca?
As far as he is concerned, Telemachus returns home as a result of Athena's warning regarding the suitor's courtship of his mother. He is concerned about their impending success and the potential implications that her remarriage has on both his family honor and his legacy in general.
From a literary standpoint, Athena is acting as an agent of the author. At this point in the story, several plot threads remain unresolved. At this point in the story, Odysseus and his son have yet to meet. Athena calling Telemachus home is a convenient way for him to run into his father on the road, a meeting that would otherwise be difficult to arrange. It also provides Odysseus with the opportunity to share his plans for revenge with his son without having to make his presence known to the kingdom in general.