Josh Marshall states it relatively simply:
The firing of a bunch of U.S. Attorneys all at once seemed strange because in the past 25 years only ten have been dismissed at a time other than a new president coming into office, and eight of those ten had a clear, publicized verifiable cause.
People who noticed this highly irregular behavior did some digging and found that a lot of them shared the trait of having pursued corruption cases against Republican politicians.
And after it was pointed out, the Republicans have done a very bad job of lying about it.
The narrative that some people wish to pursue is that the Bush administration has been nakedly partisan in its dealings with a job that is supposed to be non-partisan in nature. Yes, you generally put in U.S. Attorneys of your own party, but you also put in people who have a solid track record in the legal profession and not partisan hacks. Some people see this as consistent with Bush's history, believing that the administration has tried to pressure political-motivated outcomes in areas such as science, national intelligence, and the judiciary.