I THINK it was because he spent so many years away from home and Julie was raising 4 children by herself.
Bret used to have relationships with other women too, but I don't think Julie knew about them at the time of divorce.
His book explains better than I can.
Update: I'm completely wrong. Check this video clip. Bret chose to divorce Julie: http://www.youtube.com/watch?v=HyDAwSbdT5M&feature=related