Carol Lam resigned under threat of immediate firing.

Apparently, Bush's DOJ didn't want to give her anymore time to expand the Cunnigham investigation.

It all got quite nasty.

TPMmuckraker June 19, 2007 04:15 PM:

When a Justice Department official asked eight U.S. attorneys for their resignations last December, most of them went quietly (initially at least), agreeing to resign on relatively short notice and with no public fuss. But one U.S. attorney, Carol Lam in San Diego, had contentious private exchanges with Department officials about her end date.

An email released to Congress last week shows just how heated those discussions got. When Lam delayed announcing her date of resignation -- wanting more time to tend to several high profile cases, the expanded Duke Cunningham investigation among them --, Justice Department officials prepared to have the president fire her immediately.