What is the U.S. Government currently claiming as justification for mass firings of federal employees?

According to Rachel Maddow, the U.S. Government is attempting to justify the firing of hundreds of thousands of federal employees by claiming they've received bad performance reviews. However, Maddow firmly states that this justification is false, declaring "This isn't happening" in response to the performance review claims. The evidence suggests these mass firings are targeting government employees in crucial oversight positions rather than being based on actual performance issues.