Why did Gonzales step down now after so much resistance to doing so and public vows that it would not happen? Here's one view, but I still wonder if there isn't more to it than this:
Why did Gonzales resign?, by Sidney Blumenthal, Salon: ...Alberto Gonzales swiftly turned heel on the stage at the Department of Justice... What is he not telling about his resignation?
The true story may be something like the denouement of Edgar Allan Poe's "The Purloined Letter," which was in plain sight all along, a solution that can, as Poe wrote, "escape observation by dint of being excessively obvious..." To be excessively obvious, Gonzales' resignation, following Karl Rove's exactly by two weeks, is the shadow of the first act.
Under investigation by the House and Senate Judiciary committees for his part in the political purge of U.S. attorneys and warrantless domestic surveillance, Gonzales wandered through his appearances down winding paths of dissembling. On the U.S. attorneys, his former deputies ... contradicted him. On domestic spying, the former acting attorney general, James Comey, described then White House counsel Gonzales' attempted coup on behalf of a program Comey considered illegal through Gonzales' securing the signature of the ailing Attorney General John Ashcroft, barely able to lift his head in his hospital bed after surgery. After Gonzales offered a different account, FBI Director Robert Mueller appeared before the Senate on July 27 to corroborate Comey's version, staking his position against Gonzales' credibility. Senators called for the appointment of a special prosecutor.
In the weeks leading up to his resignation, Gonzales was undoubtedly aware of the various investigations into his activities, ... not all of them in public. As a practiced attorney, he knew that once he left government service he would become less interesting to investigators and that whatever revelations ... would have less political impact. The logic of his resignation became indisputable... But the resignation of Rove severed his lifeline to his political control agent. Without Rove, Gonzales was adrift.
From the beginning of his rise with George W. Bush until the day of his abrupt resignation, Alberto Gonzales was anointed, directed and protected by Karl Rove. ... Bush did not nickname Gonzales "Fredo," after the weak brother in "The Godfather," without reason. ...
To the extent that Gonzales was pliable he was useful. But his "remarkable journey," as he called it today in his resignation statement, was remarkable only for his unwavering subservience.
From the start, Rove and Gonzales were secret sharers. But one was "the Architect" and the other was "Fredo." With Rove's resignation, Gonzales lost the political and policy hand that had guided him all along. When the puppet master departed, the puppet collapsed in a heap.