Why would a Klingon need to wear a mask?
given what we know about Klingons it would seem cowordly to hide like that.
Fumes from mining operations?
To protect their faces from frostbite?
Those make perfect sense seeing how we learned how bitterly cold and hostile Rura Penthe is in both TUC and "Judgment(ENT)." Plus, such masks could also be a traditional/cultural thing for some guards to wear within the Empire.