It is hard for me to square "This company is a few short years away from building world-changing AGI" and "I'm stepping away to do my own thing". Maybe I'm just bad at putting myself in someone else's shoes, but I feel like if I had spent years working towards a vision of AGI, and thought that success was finally just around the corner, it'd be very difficult to walk away.
Squeezing out senior execs could be a way for him to maximize his claim on the stake. Notwithstanding, the execs may have disagreed with the shift in culture.