Michael O’Hare suffered from severe schizophrenia and was becoming difficult to work with.
Joe covered for him and got him help and promised he would never let anyone know why he left the show until after he died.
Strazinsky blamed it on the network demanding a known quantity like Bruce Boxleitner to rescue the show. Bruce took it as a compliment and JS rewrote the show to accomodate.
Nobody knew and they were all happy to see O’Hare for his role as Valen as he managed to get his meds sorted at the time.
That’s what I know. That’s the scoop I got, from drinking with Jerry Doyle one night, he pretty much confirmed said info.
Regarding Micheal O’Hara that’s pretty much what I read on a JMS blog.
Though the information on why Boxleitner was selected is new to me. I found his character - Sheridan to be the weakest and least interesting of the main characters. A very two dinensional typical TV hero character, it being a rush job more then explains it.