They were the ones investigating Dem Anthony Weiner for his child sexual predator crimes, and discovered a bunch of Clinton's classified material on his computer. They reported it to the folks in Washington, including McCabe, and they set on the information for months to try and protect Clinton...only for it to later be discovered, and Comey realizing he was in a world of hurt if he didn't report ot Congress as he said he would, made it public.
Had McCabe not tried to cover it up at first, the damage would of been less, but then again ultimately it's Clinton's own fault for having the illegal server and classified material