"review process by the board, which concluded that he was not consistently candid in his communications with the board"
OK, so they tell us he was lying, which is precisely what "not consistently candid in his communications" means.
Possible topics for lying:
* copyright issues to do with ingestion of training data
* some sort of technical failure of the OpenAI systems
* financial impropriety
* some sort of human resources issue - affair with employee
* other - some sort of political power play? Word from Satya Nadella - "get rid of him"?
Possibly the reason is something that the board members felt exposed them personally to some sort of legal liability, thus if they did not act then they would have to pay a legal price later.
It has to be pretty serious to not make it public.