Ottoman expansion brought Islam within Christendom's perceived boundaries. [causal]
By the fourteenth century, this had changed with Ottoman incursions in Bulgaria and Hungary. "Thus Islamic society was by no means as distant as some Chaucerians have suggested, or as many late-medieval Christians might have wished it to be." At this point, miscegenation and monstrous births became more prominent in stories of monstrous Muslims, who were seen as having violated the boundaries of Christian space.
Sophia Rose Arjana, Muslims in the Western Imag…, loc. 79