Before Trump was elected, the FBI was investigating Russian contacts with the Trump campaign. McCabe viewed Comey’s firing as another instance of Trump taking suspicious pro-Russian actions. Trump seemed to believe that removing Comey would end the FBI investigation into his campaign and Russian influences. McCabe, having served in the FBI’s New York field office, had spent years investigating the Russian organized crime and would recognize mob influences in Trump world.
To ensure that the evidence from the investigation of Russian contacts with the Trump campaign was preserved, McCabe rolled it into the on-going counter-intelligence investigations of known or suspected Russian agents. The effect of this action was to ensure that the Trump investigation could not be dumped without a paper trail. It also helped to make sure that a Special Counsel would be appointed.