In a waveguide consisting of a thin hollow tube, there's a cutoff frequency below which it will not work, it will not propagate energy. Above that frequency, various different patterns of E and H field can propagate, each pattern being called a mode.
If you run a wire down the middle of the tube, you add an extra mode, the coaxial or TEM mode, which conveniently goes right down to DC. Adding this extra wire changes the higher frequency waveguide modes in detail, but they still exist.
As long as you operate your coax below the waveguide-without-centre-wire-cutoff frequency, then only the TEM mode propagates, and you'll have very predictable performance, even if you bend the cable.
If you operate above this cutoff frequency, then discontinuities in the waveguide, like those that happen at connectors, or bends, can move energy from the TEM mode to other propagating waveguide modes, and back again to the TEM mode. As these other modes have different propagation speeds to the TEM mode, this will make the performance of your coax unpredictable, changing every time you bend it.