Carter page got his first FISA warrant in 2014. Long before Donald Trump was a twinkle in the eye of the MAGA set.
It seems that Mr Page was recruited as an an unwitting asset in 2013 by 2 Russian spys who have since been convicted and deported back to Russia.
https://www.nytimes.com/2017/04/04/us/politics/carter-page-trump-russia.html
That's what got him on the FBI's radar. Not his involvement with the Trump campaign. Although a normal person would have to wonder why so many of these characters ended up working for the Trump campaign.