Because of turbulent air from Mount Fuji. When air passes over the mountain, sometimes it swirls back, creating this circle of very turbulent air. The 707 flew into one of these swirls, and the plane structurally failed. All four engines were ripped from the airplane, along with the tip of the right wing, and the stabilizers. The plane entered a spin, and crashes. Many people saw the plane crash, and there is footage of the airplane taking off on its accident flight