I don't think it's planned, it's just how things tend to turn out. At least for the three startups I've worked with. If you're really good at cranking out code

I don't think it's planned, it's just how things tend to turn out. At least for the three startups I've worked with. If you're really good at cranking out code before your fledgling company runs out of money, that doesn't necessarily mean you're good at people management or leading. At a certain size, your company is going to need those skills more, and won't want to keep someone around at a C-level salary who's just slinging code. The founding eng directors I've met all separated for different reasons. One took a buyout after the company was purchased, another had a side business that became profitable and helped find his own replacement, and the last one had his role downgraded when the company hired a CTO, and just got another job.