Carter Page was suspected of having been co-opted by Russian intelligence.
Paul Manafort owed his considerable living to Ukrainian/Russian political interests, and was deep in the hole to an oligarch.
Mike Flynn was working, undisclosed, on behalf of Turkey while a senior figure on the campaign.
Now, in the last case, it's not unusual for people involved in campaigns to have overseas business or lobbying interests--even unsavory ones--but Flynn's worth mentioning because he was so prominent (named NSA and reportedly shortlisted for VP).
If Hillary had a hood like Manafort running her camapaign, you'd probably have had a stroke.