This goes back to what the original commenter was saying. The rumor is that Altman was planning to launch an AI chip startup to take on Nvidia before his ejection from OpenAI.
Unattributed sources quoted by some media outlets say that Altman has recently been fund-raising for a possible new venture with potential investors.
Given the context, I assume this was probably presented to the board, at least initially, as a personal side project outside of his duties as OpenAI CEO. If so, then the "not consistently candid" in the board's statement might be related to aspects of that, perhaps in terms of degree, focus, timing or even potential fund-raising or recruiting.
Speculating, I can imagine a scenario where Altman was working with the board to orchestrate his eventual exit to start the new endeavor or perhaps a dual role. If so, the board may have concluded Altman's outside activities were more extensive than he had disclosed and crossed the fuzzy line between discussion and exploration to execution - which could obviously create untenable conflicts of interest for Altman which the board might be required act on.
I like this hypothesis because it's consistent with the statements and actions so far as well as seeming reasonable given the massive opportunity and Altman's entrepreneurial nature. Recall that GPUs have been OpenAI's largest costs and limiting factor. In fact, as OpenAI's CEO, Altman could have argued to the board that OAI needed to vertically integrate 'down the stack' to designing it's own hardware, especially since their largest competitors already have. If the board balked at this direction as inconsistent with the non-profit's charter (and potentially even adverse to the mission of "safe AI"), Altman may have asked the board for permission to personally explore the opportunity. Assuming the board agreed, there would likely have been specific disclosure requirements of Altman as well as limits on certain activities. Such a fast-moving, dynamic environment, seems like it could lead to miscommunication and at least the appearance of material omission or even misrepresentation.
Such a breakdown in trust might result in a situation where Altman could reasonably think he's still working with the board toward a negotiated and stage-managed exit but the board could reasonably conclude the situation had gotten out of control and exceeded their agreement with Altman - requiring board action to remain in compliance with the organization's charter and their legal duties as board directors.
Since some undisclosed time period before he was even sacked according to an article in The Guardian, but it offers little else in addition to the Reuters article.