HOUSTON—Investigators Sunday worked to determine how eight people died in a crush of fans at a Houston music festival, as families mourned the dead and concertgoers recounted the horror and confusion of being trapped in the crowd. Authorities planned to use videos, witness interviews, and a review of concert procedures to figure out what went wrong Friday night during a performance by rapper Travis Scott. The tragedy unfolded when the crowd rushed the stage, squeezing people so tightly they couldn’t breathe. Billy Nasser, 24, who had traveled from Indianapolis to attend the concert, said about 15 minutes into Scott’s set, things got “really crazy” and people began crushing one another. He said he “was picking people up and trying to drag them out.” Nasser said he found a concertgoer on the ground. “I picked him up. People were stepping on him. People were like stomping, and I picked his head …