what rumor had prompted nick to move east?

Nick, like Gatsby, was from the Midwest. Nick moved to the East to start a new job and, in many ways, a different lifestyle. Nick had returned from WW1 and found the Midwest to be dull. Nick decided to go into the "bond business", rumored to be very profitable, and experience what high finance might be like. Although he planned to Live in New York city, Nick settles for West Egg which is how he comes to know Gatsby and the other filthy rich of East Egg.