Here's why:
* He was a demigod: Percy is the son of Poseidon, the Greek god of the sea. This made him a target for monsters, who are attracted to demigod energy.
* The furies were after him: The furies, monstrous creatures who punish wrongdoers, were sent to retrieve Percy because he was believed to have stolen Zeus' lightning bolt.
* He was in the wrong place at the wrong time: The bus was simply the place where the furies found him.
The attack on the bus is a pivotal moment in the story, as it reveals Percy's true nature and sets him on his journey to clear his name and recover the stolen lightning bolt.