And in Zappos' case, I get why they offered severance deals. They were embarking on a radical experiment in organizational structure, and not everyone is going to be on board with it. Rather than having a bunch of people at the water cooler talking shit about the changes, you offer them money to leave so that people who aren't on board with the changes get out of the way.
I would guess that many of the people at Zappos who took the deal were managers themselves. If you're a middle manager, you have skills around managing people but maybe not so much with doing the actual work. If your skill set doesn't align with the new job you're being asked to do (which is effectively a demotion), and the company offers you a buyout to leave, what would you do?