I have read lots of speculation out there, but to me they are all wrong. They all seem to rotate around how Picard led a rescue flotilla that tried to save Romulans and then that failed and he quitted. I think it HAS to be deeper than that. I think the Federation tried to annex parts of the Empire to "protect" Romulans against Klingons who were trying to invade their space. That hypocrisy led Picard out. I think ST Online had some good material on that, which will hopefully be used for the series. A new confrontation between Klingons and Federation seems inevitable given how differently the two powers will look at the disaster. Will see, but if writers play it well, this could be very nice.