Clearly, the four individuals who chose to leave made an earlier decision to separate their employer from their career decision making. They also came to the joint conclusion that their careers would be better served elsewhere. Without learning more about the situation it is difficult to pinpoint cause and effect but we do know that somewhere along the line, the feeling of partnership with their employer was lost.
This is an interesting case study for employers and employees alike. From an employer's perspective, detailed career dialogue with an employee can serve as a point of alignment and it has the potential to avoid situations like this. Employers are working harder than ever to understand what employees at this level want from their future and a career dialogue creates opportunities on both sides.
Of course, a career focused conversation won't always be enough to prevent employees leaving. There are some offers that can't be beaten after all. In this example, an existing career dialogue with mutually agreed priorities could have provided common ground for talks before any decision to leave was made. From that point at least the opportunities for continued partnership between employer and employee could have been discussed.