Sometimes companies hire the wrong type of person for their position. It can be hard through phone interview to get a feel about a candidate. Sometimes a person that seems to be knowledgeable can have a bad work ethic or may no mesh with the company well. They might not even have the same goals as the company and may just use this opportunity as a stepping stone.
Hiring the wrong people can lead to bad morale. Struggling to get products out, leading to lower morale, leading to slower dev time. It is a downward spiral. Manager and developer conflict can also be a real issue as both sides can have conflicting interest on what is needed to be done.