I figure he got some sort of head injury. Being a stable boy, my first thought was that he was kicked in the head by a horse.

However, there are many elborate theories around Hodor and his name. All the theories come from the theory that Hodor comes from the Norse god, Hod - sometimes spelled Hoder or Hodur.

The initial theory is that he is named after Hod, the blind giant who could harm no one without guidance. His brother was Baldur (perhaps Walder - when he was powerful and whole) and his aunt was Nanna (Old Nan). Hodor was led around by Loki who was mischievous and was tricked into killing the invulnerable Baldur. This is seen that either it means that Bran will possess him (lead him around and make him his vassal) or that Hodor will become an unwitting agent of evil and be tricked into killing someone of major significance.

The other theory is that Hodor is the name of the Great Other, and that Walder saw or was exposed to him as a child and became terrified and damaged to the point that he could never say anything else. Hodor is the god of coldness and the dark and the possible nemesis of the Lord of Light, and thus the great god north of the wall.

Most people agree that Hodor and Bran are caught up in a plot with roots in Norse mythology.