Explanation: In the Medieval period, European Christendom developed an incredibly toxic relationship with the Jewish people and faith. While tension had been present since the foundation of Christianity as a distinct religion, and persecution intermittent, Medieval Europe changed the dynamic of this persecution drastically. Whereas before, Jews were constantly pressured (or killed) on purely religious grounds, the ideological-religious foundation of Medieval European states resulted in Jews also being shunted into 'unfitting' work.
One of the core religious-ideological principles of the High Middle Ages was the notion of the Three Orders - those who worked, those who preached, and those who fought. Merchants and moneylenders (with Christian moneylending in particular being restricted by the Catholic Church), one might note, fit neatly into none of these, making them a sort of ideological outcast. Since Jews were already religious outcasts... you may see where this is going.
The level of restriction upon Jewish economic activity was utterly insane. In many places, they were not only forbidden from owning land, but forbidden from working land even as tenant farmers. Imagine being told "We are not going to allow you to be a peasant". After being shunted into mercantile work, many became rich - which then resulted in the Jews being decried as 'greedy' - a convenient excuse whenever locals or notables wanted to murder or expel some Jewish communities and seize all of their worldly goods. Thus making Jews both a key part of the functioning of European economies and an intensely persecuted minority. What a... lovely situation.
While there was considerable tension and antisemitism in Medieval Islamic polities, it was nowhere near as intense or as rigid as it was amongst European Christendom, and for that reason a great many Jews intermittently fled after periods of European Christian persecution to Muslim-controlled polities like Al-Andalus, Egypt, and later, the Ottoman Empire. Later, Poland would deploy a limited form of religious tolerance which would make it also a major destination for expelled Jews from other Christian polities.