Another is that the board could have threatened to sue Kalanick for violating the loyalty and duty obligations that corporate officers have to shareholders - e.g. for suppressing systemic sexual harassment, withholding details about the Otto acquisition, etc.
He may not, but I have seen a board threaten a CEO in a situation like that. It never came to a lawsuit, the CEO stepped down but this sort of thing definitely can happen depending on how bad the situation is.
Investors sue all the officers all the time. United Airlines, Ebay/Craigslist, Skype, non-profits, etc.
Suppressing systemic sexual harassment violates his duty to report management problems to the board. Same with any meetings with Otto's founder a week prior to them leaving Google and founding Otto.
Another is that the board could have threatened to sue Kalanick for violating the loyalty and duty obligations that corporate officers have to shareholders - e.g. for suppressing systemic sexual harassment, withholding details about the Otto acquisition, etc.