Elon was irritated that he was behind in the AI intellectual property race and this narrative created a perfect opportunity. Not surprised in the end. Tesla effectively did the same thing - "come help me save the planet" with overpriced cars. [Edit: Apparently Elon has left OpenAI but I don't believe for a second that he will not participate in this LP]
(I work at OpenAI.)
The board of OpenAI Nonprofit retains full control. Investors don't get a vote. Some investors may be on the board, but: (a) only a minority of the board are allowed to have a stake in OpenAI LP, and (b) anyone with a stake can't vote in decisions that may conflict with the mission: https://openai.com/blog/openai-lp/#themissioncomesfirst