  Recent experiments show a strong rotational-diffusion enhancement for
self-propelled microrheological probes in colloidal glasses. Here, we provide
microscopic understanding using simulations with a frictional probe-medium
coupling that converts active translation into rotation. Diffusive enhancement
emerges from the medium's disordered structure and peaks at a second-order
transition in the number of contacts. Our results reproduce the salient
features of the colloidal glass experiment and support an effective description
that is applicable to a broader class of viscoelastic suspensions.
