http://en.wikipedia.org/wiki/Michael_O%27Hare#Illness
As Babylon 5 creator J. Michael Straczynski describes it,[3] during the filming of the first season of Babylon 5, O'Hare began having paranoid delusions. Halfway through filming, his hallucinations worsened. It became increasingly difficult for O'Hare to continue work, his behavior was becoming increasingly erratic and he was often at odds with his colleagues. O'Hare sought treatment for his mental illness, but feared that, as the main character of Babylon 5, taking an extended medical leave of absence would destroy the show just as it was getting off the ground.
Straczynski offered to suspend the show for several months to accommodate O'Hare's treatment for his mental health; however O'Hare refused to put so many other people's jobs at risk. Straczynski agreed to keep his condition secret to protect O'Hare's career. O'Hare agreed to complete the first season but would be written out of the second season so that he could seek treatment. He reappeared in a cameo appearance early in season two and returned in season three for the double episode "War Without End", which closed his character's story arc. He made no further appearances on Babylon 5.
Although his treatments were somewhat successful, he was never fully cured. Upon his return to Babylon 5, Straczynski promised again that he would keep his condition secret to his grave. O'Hare told him to "keep the secret to my grave", pointing out that fans deserved to eventually learn the real reason for his departure and that his experience could raise awareness and understanding for people suffering from mental illness. On May 25, 2013, eight months after O'Hare's death, Straczynski fulfilled his promise and finally revealed the reasons behind O'Hare's departure from Babylon 5 at the Phoenix Comicon.
