An uneducated guess is that the ‘martial race’ concept was an H.R practise of the medieval era for the Brits to boost their colonial military. A community with a culture of sending men to combat had the military as a prime occupation and trained up its youth for the role.
Many of the ethnic groups which were thus designated-Gurkhas and other hill communities, Jats, Sikhs and Pathans still figure prominently as far as numbers go in their respective armed forces.
Interesting stuff.