According to Deadline, Marvel Studios has already begun looking for a new director -- and they have to do so pretty quick -- as Thor 2 is scheduled for a November 15, 2013 release.
No specific reasons were stated as to the departure, but it is rather interesting. There have been previous rumors that the reason for Jon Favreau not returning to helm Iron Man 3 was due to Marvel Studios' "involvement" in Iron Man 2. Mickey Rourke has even gone on record recently, while promoting Immortals, stating how Marvel interefered with his character, Whiplash. Rourke was none too happy to say the least.
Cosmic Book News will be sure to update on Thor 2 when more becomes available.