Page was on the FBI radar as early as 2013. He was caught up in surveillance of a Russian National who dropped his name. The FBI interviewed him and concluded he didn't know that some of his Russian contacts were spies. Law enforcement officials say that there was more information aside from the dossier in the application to surveil Page.
For all we know, Page didn't do anything illegal and he's just a maroon. Hopefully the application for a FISA warrant for Page will be released, but in either case, the FISA warrant was reauthorized several times, the last two under Trump's own Justice Department.
What does that have to do with Trump?