The primary thing to know is that the actor who played Sinclair in season one, Michael O'Hare, suffered from mental illness.

https://en.wikipedia.org/wiki/Michael_O%27Hare

His treatments were only partially successful. He reappeared in a cameo appearance early in season two ("The Coming of Shadows") and returned in season three for a two-part episode ("War Without End") which closed his character's story arc. At that time, Straczynski promised O'Hare to keep his condition secret "to my grave". O'Hare told him to instead "keep the secret to my grave", arguing that fans deserved to eventually learn the real reason for his departure, and that his experience could raise awareness and understanding for people with mental illness. He made no further appearances on Babylon 5 but continued to support the show and appeared at conventions and signing events until his retirement from public appearances in 2000.

On September 28, 2012, Straczynski posted that O'Hare had had a heart attack in New York City five days earlier and had remained in a coma until his death that day.[48] Eight months later, Straczynski revealed the circumstances of O'Hare's departure from Babylon 5 at a presentation about the series at the Phoenix Comicon.