I think is has to do with the fact that they were so set apart from society for so long. Jews were forbidden to demand usery (interest) from one another, as were early European Christians. Therefore, Jews often went into banking and prospered by lending money to non-Jews. That caused the old resentment of debtor to lender (Just like I resent Sallie Mae for my astronomical student loan).
Jews were forbidden to go into many professions, so they became very good at the few they could practice and often stayed in the same profession generation after generation. They prospered. Often, this conspicuous properity led to the false assumption that they were corrupt and stealing from the non-Jews. This tended to become a sort of mythology not countered for many generations until it tended to boil over into anti-Jewish violence in many places around Europe.
They set themselves apart from the society. There is no universal law that various people will target Jews for no reason.
However by the usury business hangs an interesting tale.
Initially usury was forced on them because an economy couldn't exisd without it. As a consequence the Jews became the bankers of the time. When the nobility built up a big enough debt to the Jewish bankers they would find some sort of execuse to boot the Jews out and thus get rid of the loan they owed the Jews. This was one of the main reason they wer expelled from England and Vienna. In other instances they were just outright killed or forcibly dispossed of their wealth.
The Catholic Church made usury "legal" after pressure from the nobles who saw what a lucrative business banking could be. One of the reasons Oliver Cromwell invited the Jews back to England was the Jewish bankers kicked out of England had help make Holland a power and he wanted their talent for England.
And for you Yanks who might be interested Jewish bankers in Europe funded the Revolutionary War for you which pi$$ed the British off no end.