Not sure why this sounds so incredulous. I have left (or even better, been laid off and happy to collect severance) more than once. It's not strictly "random" of course, it usually comes down to boredom and need for change but the bottom line is that indeed nothing could have been done.
Yeah, there's also "an excellent opportunity arose," and "time for a new challenge," or "looking for a bigger/smaller, riskier/stable team/company/toaster/product.
