Without having the lesion to examine, I will venture a quess that this is the result of a mixed anaerobic infection. Could be a fungal infection but my first guess is anaerobic. The massive swelling is indicative of gas production common with anaeobic growth. These organisms are extremely sensitive to molecular oxygen and are capable of growth only in deep tissue pockets. In this location the gas is trapped and the surrounding tissue swells; gaseous gangrene, if the right bacterium is present.
I think it highly unlikely that the disease is contagious and therefore would be of little consequence to the trapper.
Now as for the fur value, the tails on gray fox, at least last year, were very important to the value of the pelt. Seems current fashion demands are calling for the tails for trim.