John Kelly has a pretty succinct explanation in The Great Mortality, his book about the Black Death. His notion was the Jews occupied a unique elite stratum of society, in part because they weren't affected by Christian laws against usury. Throughout Western Europe, Jews specialized in moneylending and banking, and they became villains when they were forced to foreclose upon property.

"Increasingly, in villages and forest clearings," Kelly writes, "men and women told one another: Perhaps if we kill all the Jews, the plague won't come to our village. Even if the plague did come, with the Jews dead, at least debts to Jewish moneylenders would be canceled. Later, after the violence subsided, one chronicler would write that the 'poison which killed the Jews was their wealth.'"

This is only one factor, but it's a significant one in the rise of antisemitism in Europe during the Middle Ages.

I'm not one of the scholars here, but since you didn't get much response . . .

Yes, Jesus was Jewish, but the gospels make it unmistakable that the Jewish Sanhedrin were responsible for Jesus's death, even going so far as to have Pilate ceremoniously wash his hands of the whole affair. Writings of the early Christian era take this even further, with the apocryphal Acts of Pilate having him go so far as to become a Christian himself.

Other early Christian writings were quiet horrid, IMO, towards the Jews, saying they were too ignorant to recognize their own savior. Who comes to mind here is Justin Martyr, particularly in his Dialogue with Trypho.