Money lending used to be banned among Christian-based laws so Jews served as money lenders. Since Christianity viewed this as sinful, Jews were sometimes seen as controlling the wealth in a sinful way (maybe you might call them the Goldman Sachs executives of their day, in popular interpretation.

Perhaps because of this (someone else can speak to the reasons, maybe must cultural), they tended to form tight knit communities around the world and could be seen as separate from the rest of the population (different language, dress). So in that way like any other minority group they could be scapegoated. And in the case of the Jews they actually did hold significant power in some places which made it easier to blame them for things.

edit: I should add banning money lending is terrible economics, which is why the Jews were so successful capturing that market. They were a necessary "evil" even if that is what you considered evil at the time.