You don't fire your CEO and call him a liar if you have any choice about it. That just invites a lawsuit, bad blood, and a poor reputation in the very small circles of corporate executives and board members.
That makes me think that Sam did something on OpenAI's behalf that could be construed as criminal, and the board had to fire him immediately and disavow all knowledge ("not completely candid") so that they don't bear any legal liability. It also fits with the new CEO being the person previously in charge of safety, governance, ethics, etc.
That Greg Brockman, Eric Schmidt, et al are defending Altman makes me think that this is in a legal grey area, something new, and it was on behalf of training better models. Something that an ends-justifies-the-means technologist could look at and think "Of course, why aren't we doing that?" while a layperson would be like "I can't believe you did that." It's probably not something mundane like copyright infringement or webscraping or even GDPR/CalOppa violations though - those are civil penalties, and wouldn't make the board panic as strongly as they did.