How could Orthodoxy have existed in pockets of Western Europe? From what I understand, though there were bitter theological differences between East and West, and political rivalries that most dramatically culminated in the Sacking of Constantinople, there wasn't the sort of internecine bloodshed and persecution between adherents of the two churches like there was between Protestants and Catholics. Or was there? There probably wasn't too much interaction between the two churches in the Middle Ages anyway, Frankish imperialism in Anatolia aside.
So how could there be significant minorities, or at least notable communities, of Eastern Orthodox believers in western Europe during the Middle Ages? Greek or Russian traders? Certainly there were Catholics going east for trade or crusade, but what could lead the Orthodox to go west?