The specific event was that the Byzantine Empire was fighting against the Seljuk Turks in Anatolia, so they asked for help from western Europe. Pope Urban II started recruiting people, and...somehow...everyone got the idea in their heads that they could go all the way to Jerusalem. Unfortunately we're not entirely certain how that happened!

Here are some previous answers that might help explain in more detail:

Who started the Crusades? (Each one) by /u/Ambarenya

Were the Crusades a defensive move? by /u/thejukeboxhero

Crusades- Offense or defence? by me

Does anyone know of any contemporary sources detailing the motivations of crusaders on the first crusade in 1096? also by me