Note: This understanding is based primarily on Part One of In the Year 1096.
The Jews had, officially, been doing well in the Rhineland. The civil and church officials liked them, for having Jews in the town meant more business in the town, and more taxes, too. In fact, when some Jews from Mainz wanted to move to Speyer, the bishop of Speyer offered them very comfortable conditions to which to move, including a fortified section of the city (Chazan, p. 6f.).
However, all the business the Jews did competed with someone, of course, and that someone was the burgher. Further, all the rights afforded the Jews were lost opportunities for the burghers. (For example, the Jewish semi-autonomy meant the burghers had no judicial jurisdiction over the Jews in Speyer.) The burghers and the general populace of the towns the Jews were in did not particularly care for the Jews, but could do little about them, considering that the officials liked them (Chazan, pp. 11, 17).
In 1095, Pope Urban called for taking Palestine back from the infidel Moslems. Many Christians, who believed the Jews to be Jesus' killer, decided that, instead of waiting until they got to the Holy Land to avenge themselves on the infidels, they could do so while still in Europe. After all, whom better to avenge themselves on than the killers of Jesus?
It should be noted, however, that (at least according to Chazan, pp. 55-6) Urban did not intend any antisemitic uprising. He was concerned with the lands to the east, not with the deicides in Europe. Indeed, the official Crusaders hardly touched the Jews; it was the disorganized bands who inflicted the most damage, and who set the stage for hundreds of years of pogroms and other massacres.