  We revisit the effects of short-ranged random quenched disorder on the
universal scaling properties of the classical $N$-vector model with cubic
anisotropy. We set up the nonconserved relaxational dynamics of the model, and
study the universal dynamic scaling near the second order phase transition. We
extract the critical exponents and the dynamic exponent in a one-loop dynamic
renormalisation group calculation with short-ranged isotropic disorder. We show
that the dynamics near a critical point is generically slower when the quenched
disorder is relevant than when it is not, independent of whether the pure model
is isotropic or cubic anisotropic. We demonstrate the surprising thresholdless
instability of the associated universality class due to perturbations from
rotational invariance breaking quenched disorder-order parameter coupling,
indicating breakdown of dynamic scaling. We speculate that this may imply a
novel first order transition in the model, induced by a symmetry-breaking
disorder.
