This paper focuses on secondary instability mechanisms of high-speed boundary layers over cones with a circular cross section. Hypersonic tran7sition investigations at Mach 8 are performed using Direct Numerical Simulations (DNS). At wind-tunnel conditions, these simulations allow for comparison with experimental measurements to verify fundamental stability characteristics. According to Linear Stability Theory (LST) the boundary layer is most unstable to second-mode two-dimensional waves. Our investigations indicate that secondary instability mechanisms involving two-dimensional waves appear to be of lesser importance in the nonlinear stages of breakdown to turbulence in spite of the high amplification rates of the two-dimensional primary disturbances. Instead, second-mode oblique waves at small wave angles, which are almost as amplified as second-mode two-dimensional waves, dominate the nonlinear behavior.