It was for political reasons. How could all 93 be held to be incompetent in the first couple of weeks of his administration?
Brit Hume was either being disingenuous, or he's misinformed. Bush did replace all the attorneys within six months of taking office, just as all presidents do. Political appointees, such as the federal attorneys, are replaced with each new administration. There is nothing wrong or unusual with that.
The issue here is that the attorneys who've been fired have charged that they were intimidated or threatened, the AGs chief of staff lied to Congress about it, e-mails are coming out that show the process was not handled appropriately, and Gonzales tried to replace them with interim appointments that would not be subject to Senate approval.