Power is a fuzzy thing. You can think about power as being distributed across lots of different entities (the board, CEO, senior execs, investors, rank and file employees, etc) with some having more concentrated power (eg the board) than others (eg individual employees). However, if you create a situation (eg lots of employees decide to walk out in support of the ousted CEO) that can aggregate enough power to overcome any other single entity. That seems to be what is happening here.
It does not matter that the board has the legal power to do whatever they want eg fire the CEO. If the investors and key employees that keep the company going walk away, they end up with nothing so they might as well resign and preserve the organization rather than burn the whole thing down.