The Republicans were attempting to crack down on alleged voter fraud in locations that had high Democratic voter turnout, despite the fact that no prosecutable cases of such fraud have turned up. The American Center for Voting Rights was an apparent think tank (which formed right before the hearings started) which offered sound bites and alleged research documenting the putative problem.
Several of the fired U.S. Attorneys were individuals who had refused to go after weak voter fraud cases against Democratic candidates in locations where the Republicans and ACVR were also trying to make it more difficult to vote and reduce Democratic voter turnout.
Check out the Salon.com article.