The specific incident was the Charlottesville protests and consequent murders by white supremacists. They had stepped in to defend Jason Kessler's argument against moving the protest (where the city wanted to move the protest because they correctly identified the violence that would occur) and made a ton of PR gaffes during this time. As a result the ACLU ended up losing a ton of support both externally and internally because they had played a part in an event that was obviously going to turn violent.