According to the paper:
Colleagues said Dannehy is not a supporter of President Donald J. Trump and has been concerned in recent weeks by what she believed was pressure from Barr - who appointed Durham - to produce results before the election. They said she has been considering resignation for weeks, conflicted by loyalty to Durham and concern about politics.
Colleagues of Durham who know nothing about the investigation say they believe Durham is under pressure to produce results before the election:
Durham is notoriously circumspect and neither he nor members of his team have revealed anything about the direction of their work. But Durham associates, none of whom have specific knowledge of the investigation, have said recently that it is their belief he is under pressure to produce something - perhaps some sort of report - before the presidential election in November.
Despite his mild public demeanor, Barr is a notoriously hard charger. Pressure? I imagine so.
UPDATE 1: Are there ever legitimate political reasons to push to come up with results when you're investigating the biggest political scandal and crime in the history of our nation? I believe so. Simply making up for time lost to the pandemic is a legitimate reason for pressure--justice delayed is justice denied, and the evidence of criminal injustice that was done to Trump and to others is everywhere to be seen. Restoring some measure of integrity to our public life is not just a matter of criminal justice but of restoring political health to our polity.
UPDATE 2: I highly recommend SWC's tweets on why Dannehy might have left. Basically, he goes over her career, figures she may not actually have tried any cases since around 2010, is around 60 years old, and may simply not feel up to putting in what he figures would be 18-24 months minimum taking a case to completion--taking appeals into account. She may have a good earning private gig lined up and can put in a few years doing that before retiring.