What are the root historical causes of anti-Semitism (esp. before Jews were blamed for Christ being crucified)? Are any of these causes justified, or are they propoganda and myth?
I do not believe that there was what we would call anti-Semitism before there was the idea that the Jews were to blame for the crucifixion of Christ. The Romans, for example, disliked Judaism because Jews would not even make a pretence of worshipping Roman gods, but I would not call that anti-Semitism because the Romans disliked Christians for the same reason.
To me, a non-religious cause of anti-Semitism was jealousy and distrust of the "other." Jews were always seen as different because of their different dietary laws and such. They often chose (and were often forced) to live apart from the rest of the people in the countries where they settled. They also tended to be pretty successful in business and professional life.
Because of this, they were seen as different and there was some amount of jealousy because of their success. These are non-religious for anti-Semitism.
One of the main excuses (I don't like to use the word causes) that people have used in ancient times to support prejudice against Jews is the idea that the Jewish people are God's chosen people, an idea that was discussed often and at length in the Old Testament and many of the writers and prophets during the time of the Old Testament. There is a backlash against the idea of one people being chosen by god while others are not based simply on birthright. People used this as motivation/justification long before the time of Christ.
Another historical cause is the invasion by the Hebrew people of the area of Palestine after their captivity and subsequent exodus from Egypt. The various peoples who were displaced certainly didn't have very many positive feelings about the people they saw as invaders who claimed a right to the land given them by god.