Rogue forces within the FBI have been working on the Clinton Investigation since at least August 2016
Read between the lines.
People within the DOJ caught wind of FBI investigators asking around.
McCabe couldn't credibly stop it without it appearing obvious, but he could hamstring them - like not letting them issue warrants or question people.
https://www.realclearpolitics.com/articles/2018/04/23/the_hidden_bombshell_in_the_mccabe_report_136882.html