In this paper, using aerodynamic numerical analysis, we investigate mechanisms causing disk flutter in hard disk drives. A computer simulation model of three stacked disks was calculated using the finite-difference method with the LES model of turbulence. As a result, secondary airflows were found to move upward and downward though a shroud gap around the outer edge of the disk. This inference from this discovery is that a phase difference of spatial pressure patterns between the top surface and bottom surface of a disk causes disk flutter.