So when McCabe was acting Director of FBI, in between Comeys firing and appointment of Mueller in Spring 2017, McCabe opened up criminal investigations into Sessions (perjury) and Trump (obstruction of justice).
Rosenstein is going to come out of this thing intact I believe, he was working with trump to get McCabe off the investigation.
Source WaPo article today
https://www.washingtonpost.com/amphtml/world/national-security/rosenstein-mccabe-feud-dates-back-to-angry-standoff-in-front-of-mueller/2018/10/10/8a7e99fe-ccac-11e8-a3e6-44daa3d35ede_story.html?tid=ss_tw-amp&__twitter_impression=true