The way counterintelligence works in the U.S. is that there is a sort of ongoing assumption that countries like Russia are actively, and at all times, trying to spy on us, and hurt us in various w
ays. So, from the perspective of U.S. national interests, think of it like a crime that never ends. The reason why the crime never ends is that, in the case of Russia, the government, no matter who is in charge, continues to exist without respect to the involvement of individual human beings. Further, the interests of the Russian government do not cease to exist, ever. Also, the interests of the U.S. government do not cease to exist ever. In both cases, the interests of the governments may change, but they do not cease to exist. Short of a change of mentality on part of the people who manage the Russian government, the Russian government will never stop spying on us, and hurting us in various, in order to further their own interests. The spying will continue indefinitely until the Russian government changes its strategies.
So, therefore, when the USIC picks up signals or indicates that an intelligence operation of some sort is being conducted against the U.S., the USIC has an obligation to the Republic and its people to investigate it. And the thought that entered the minds of people like McCabe, Comey, and Strzok when this first all happened, was not, "OMG what is Trump doing...", instead it was, "OMG what is the Russian government trying to do us now." And when they investigated people like Manafort, Page, Papadopolous (whose name I can never remember how to spell correctly), they are not initially investigating them as if they were traitors. They know the Russian government is good at manipulating people. They know that the Trump campaign staff are the kind of people who can easily be manipulated.
So, it's from that frame of reference, that the FBI went and launched Crossfire Hurricane. Specifically, it was things like Papadapolous bragging to Australia's lead diplomat, that he was in cahoots with the Russian government to help the Trump campaign win.
As time progressed, and the USIC acquired more information, it became clear that the Trump campaign was doing really strange and suspicious things. I think the USIC eventually became suspicious of Trump himself. And the real turning point for the USIC and also for U.S. citizens who were not supporters of Trump, was when Trump fired Comey for not ending the investigation into Flynn. It was when Trump fired Comey that the Special Counsel's investigation began. That was thing that really made people suspicious.