A lot of other theories don't match because the board was in an extreme hurry to get him out (voting him out in the middle of the night, not even waiting for markets to close to announce it), must have proof of serious misconduct and the need to immediately distance themselves from Sam (otherwise the language would not be as bridge-burning as it is) and a major financial or technical failure seems unlikely, since the CFO remains and the CTO was even promoted to interim CEO - they seem to be trusted, still, so it must have been something Sam did on his own.
Leaking/Selling the full models matches, as this would violate OpenAIs non-profit terms, would be something Sam could do without the C-Suite being implicated and (given the data contained in the training set) might even be legal hot water, justifying this drastic reaction and immediate distancing.