On this page - http://atheism.about.com/library/FAQs/christian/blchron_xian_crusades02.htm
May 06, 1096 Crusaders moving through the Rhine Valley massacre Jews in Speyer. This is the first major slaughter of a Jewish community by Crusaders marching to the Holy Land.
May 18, 1096 Crusaders massacre Jews in Worms, Germany.
It claims that many Jewish communities were massacred. Why were they attacked, and how many were killed by the Crusaders?
Because the Crusade armies had been raised to go fight non-Christians. They'd been given Church sanction, and even blessing, to kill non-Christians. So they're on their way to the Holy Land, ready and eager to deal out some holy justice to the nonbelievers, and not yet worn out by months of marching. Oh hey look, there's a village of filthy nonbelievers. Why don't we just wave cheerfully as we go by?
It's really that simple. If you want to learn more about this sort of stuff, I'd recommend Gavin Langmuir's stuff, or In the Year 1096: The First Crusade and the Jews by Michael Chazan.