Spent the last hour or so reading articles on her. She was definitely active in getting congress to renew the 702 surveillance act, for which Sessions heaped praise on her.
Can't find a solid reason for her leaving, other than the 'timing' Q mentions.
So this is what made the most sense (so far).
Report: Rachel Brand quit the DOJ partly for fear that she’d be put in charge of Russiagate
…If you’re wondering who’s next in line now to fire Mueller should the word come down from 1600 Pennsylvania Avenue, there’s an answer. And he’s a Trump appointee.
By default, under an obscure statute known as the the Vacancies Reform Act of 1998, Brand’s temporary successor as the “acting” associate attorney general is her principal deputy, Jesse Panuccio. That same statute would also allow the president to choose someone else to serve as the “acting” AAG on a temporary basis for up to 210 days; the pool of individuals from which the president could draw in this case includes individuals already holding Senate-confirmed positions elsewhere in the executive branch (like EPA administrator Scott Pruitt) or senior civil service lawyers in the Justice Department, specifically…
However, because Panuccio is not Senate-confirmed, he would not act as attorney general (or deputy attorney general) if those offices were also to become vacant. Instead, under the Justice Department’s own succession statute and guidelines implementing that law issued by Attorney General Loretta Lynch in November 2016, Brand is replaced in that line of succession by Solicitor General Noel Francisco (the solicitor general is the nation’s chief legal representative in front of the Supreme Court).
Francisco is in the same boat as Brand is. He’s young (just 49), has impeccable credentials (former Scalia clerk, later a top lawyer in the White House and DOJ), and holds a glamorous job at the Justice Department right now. The solicitor general is perennially a top-tier candidate for Supreme Court vacancies; Francisco would obviously be in the mix given his age. But now, suddenly, all that’s standing between him and a nasty political jam is Rod Rosenstein. If Trump demands that Mueller be fired and Rosenstein resigns, the ball is tossed to Francisco to decide what to do. If he resigns too, suddenly the DOJ will have lost its deputy AG, its associate AG (due to Brand’s resignation), and its solicitor general. The department would be gutted. But if he doesn’t resign and carries out the order, a la Robert Bork, to fire Mueller, he’ll be a mortal enemy of Democrats forever. His future in government might be over, however personally grateful to him Trump might be. I wonder if Francisco’s looking around for any “dream jobs” in the private sector right now. I wonder how many other DOJ lawyers are too.
https://hotair.com/archives/2018/02/12/report-rachel-brand-quit-doj-partly-fear-shed-put-charge-russiagate/
It makes sense. That's all I got.