Read the original article. They weren't investigating him because they didn't like him. They legitimately had multiple reasons to think the unthinkable. He fired Comey and said on NBC that he did so to shutdown the Russia investigation. They were aware of some of his campaign's ties to Russia. That is some pretty strong evidence to at least start an investigation.
Agents and senior F.B.I. officials had grown suspicious of Mr. Trump’s ties to Russia during the 2016 campaign but held off on opening an investigation into him, the people said, in part because they were uncertain how to proceed with an inquiry of such sensitivity and magnitude. But the president’s activities before and after Mr. Comey’s firing in May 2017, particularly two instances in which Mr. Trump tied the Comey dismissal to the Russia investigation, helped prompt the counterintelligence aspect of the inquiry, the people said.
https://www.nytimes.com/2019/01/11/us/politics/fbi-trump-russia-inquiry.html