I still hold true to my theory that it's possible that Thor may have learned about his Brother's survival sometime before the events of "The Avengers" and that he was probably tracking him towards the other end of the galaxy, after having potentially heard rumors of his dealings with the so called "gangsters" of the nine realms.
Thor probably got into a scuffle with someone or with a group of beings which somehow resulted in him being transported to Earth.
The reason why I say that he was transported to Earth by means other than his own capabilities is that I don't think Thor has learned on how to travel between realms on his own (with the usage of his hammer) as of yet. Nor do I think that the Bi-Frost has been completely repaired.
IF Thor was capable of traveling between Realms on his own accord, then I would imagined that he would have brought a army of Asgardians to Earth to help defend it instead of just relying on his Avengers' Teammates.