[Managers at Amazon are] expected to lose, either voluntarily or through termination, a specific number of employees every year. If you don't, you're expected to make up for it the following year. Managers are even evaluated using this metric, known as "unregretted attrition rate" (URA). Basically, it's the number of people you wouldn't be sad to see leave the company.
Amazon managers are hiring people they otherwise wouldn't, or shouldn't, just so they can later fire them to hit their goal. That completely defeats the point since--if the metric is based on sound business principles--there are people keeping their job who shouldn't, at the expense of the sacrificial lamb.