From what I recall, JMS originally offered the explanation that:
1. He had written the character of Jeffrey Sinclair to be something of a warrior-philosopher (which makes a great deal of sense if you are familiar with the character's eventual fate); with considerable emphasis on the 'philosopher' component in the first season.
2. From the second season forward, Sinclair would adopt a more action-oriented role, and drive the main story arc forward.
3. He hadn't laid the necessary groundwork for this change; and having written himself into a corner, JMS opted to exercise one of his famous 'trapdoors' and switch Sinclair out for John Sheridan.
The reasoning wasn't wholly implausible (imagine, for instance, Sinclair deploying nuclear weapons with the same sort of joyful abandon as Sheridan); and the timing happened to coincide with the introduction of Hotshot Network Note Warren Keffer (which lent some credence to the idea that TNT had a hand in the change)
To give full credit to JMS: not only did he protect Michael O'Hare's reputation (and kept the actor's mental health struggles in confidence until after his passing), but he did so by offering up a cover story that placed the blame entirely on his own shoulders.
If I had a nickel for every time a 90s sci-fi show had to write out a character who had visions and other mental power things, because their actor developed severe mental health issues, only for the producers to lie about why they'd been written out for decades afterwards? And they came back once or twice for a cameo?
I'd have 10 cents, but it's still weird that it happened twice.
(Kes played by Jennifer Lien on Star Trek: Voyager and Jeffery Sinclair played by Michael O'Hare on Babylon 5)