This was tens of thousands of peasants in a poorly organized militia, moving through unfamiliar territory with the noble goal of doing something "Christian"; unfortunately, this mood of theirs made them see any non-Christian as a target.
This anti-non-Christian mindset motivated them to attack Jews. There were some specific factors we might consider. One was the need for money: they were peasants, and travel expenses (food, shelter) were beyond their meager personal means. Thousands of people crossing unfamiliar land was always stressful for the natives. Jews were a popular source of quick funds by simply stealing from them or even killing them.
Also, to the Christian citizens of France and Germany, Jews were responsible for the Crucifixion of Christ, and so clearly were the enemy of Christians. This was the beginning of "Crusade fever" that inspired anti-Jewish violence for the next couple centuries at least.
Another factor was the presence of Count Emicho of Leiningen. While Peter's people were likely to threaten Jews in the towns through which they passed, bribes of money smoothed this over and people were usually unharmed. He joined Peter the Hermit and brought along his own history of attacking Jews. Emicho shortly before all this was known to attack Jews and force conversions on them.
Peter supposedly carried with him a letter from the Jews of France requesting of the Jews of the Rhineland that they support the Crusade. A Jewish chronicler of the mid-12th century, in the Solomon bar Simson Chronicle, records that Peter's arrival caused such fear in a town that the Jews readily supplied him with his needs.
Jewish communities in Mainz, Speyer, and Worms were ransacked before the Crusaders moved on. These three prominent populations of Jews banded together to enact a series of rules and policies concerning interactions between Jews and Gentiles. We may continue Peter the Hermit's People's Crusade a little later, but first let's take a look at the Enactments of SHU"M.
