Why does concentrated HCl fume?
In the case of $\ce{HNO3}$, according the abstract of this paper$^\dagger$, nitric acid vapors act as nucleation sites for ambient water, causing the mists/fumes that are observed above concentrated nitric acid solutions:


  The gas phase reaction [forming nitric acid from other reagents] supplies $\ce{HNO3}$ nuclei on which the water vapor readily condenses to form strong nitric acid mist droplets.


According to this book$^\ddagger$:


  Hydrochloric acid has thermodynamic properties rather similar to those of nitric acid.


So, I would hypothesize that the physical chemistry phenomena of evaporation and hygroscopic nucleation are responsible for the fuming of $\ce{HCl}$.

$^\dagger$ Goyer, J Colloid Sci 18(7), 616 (1963)

$^\ddagger$ Kulmala and Wagner, "Nucleation and Atmospheric Aerosols," 1996, p591.