Cutoff or Corner Frequency for Coax Cable
Below the cutoff frequency, coaxial cable will only support the TEM mode.

Above the cutoff frequency, it will support other waveguide modes.

In a perfectly uniform cable, a propagating TEM wave would continue to propagate, without exciting any other modes, so would appear to be working as expected.

In a real cable, any bend, connector discontinuity, surface roughness, would couple some energy from the main TEM mode into other modes. As they have different speeds and different losses, the cable would appear dispersive. 

Unlike in a cable below cutoff, where the phase and loss are only very weakly dependent on its bend radius, the change in coupling to other modes, and so the apparent change in phase and loss would be strongly, and apparently randomly, dependent on any cable flexing. It's this instability in the response of the cable above cutoff that is the real barrier to using it in this frequency range.