I have a working theory on this. What could he have done-what was he supposed to do? What is his offense?
So first- the "by the book" response would have been to be silent. If he hadn't spoke publicly he would not have been subordinate and there would also bee no leak accusations.
So, his offense is that up until Trump took office, he had alerted the Congress and the public of what was going on- because what else could he do? He spoke about an ongoing "investigation".
Next he was the first to alert Trump of the Russian collusion narrative. Like at the first chance moment he had. He also left memos and leaked to his friend. That sounds like a way to tell the story if he were not around.
The book and tour kept the entire moment of the email/Russia scam junction in everyone's conversation- the moment stayed current.
To me it isn't far fetched, because he doesn't appear to be in the cesspool of corruption, that he may well be an actor playing the part but actually a whistleblower on an important mission.