How did this method of sex determination come to be?

The Khanghaji originates from an evolutionary stem that happened upon the Aethini as a method of selecting the strongest among a group of Aethini to become complete males so the entire group had the best chance of survival. In essence, the most physically capable went on to become the best hunters and fighters. Additionally, by having all Aethini be born male, the species was provided with a pool of capable younger individuals that could better assist with hunting and other physically demanding tasks. At this stage, the young males would fight mostly according to instinct and carnal urges. It is an evolutionary marvel that quickly allowed Aethini to become the dominant species of their planet. When paired with greater intelligence, it allowed the Aethini to continue to evolve into a biologically formiddable species.

Once the species had evolved from its primal roots, the advent of a prehistorical society really allowed their intelligence to develop which in turn led to the emergence of a theorised ritual which is generally accepted as the first instance of what the Khanghaji is today. This ritual would have been used as a coming of age ceremony for those young males who reach adulthood to find their place in the tribe. Most likely, this ritual combat would have been spectated and celebrated as a monumentous event in someone's life.

As the tribe grew in size, this was managed differently by a male coming of age being given a special earring to wear. Specimens of this earring and stone carvings describing its use have been discovered at archaeological sites. This earring was provided to them by a family member and they would wear it to signify to others that they were of age and could be challenged to Khanghaji. This is, in essence, is how the tradition is carried on to this day. Except the earring is standard issue and provided by a government agency that tracks the population.

As a young male on modern day Aeterna, upon reaching the age of 18 you will have the earring sent to your address. It is your choice when to begin wearing it, but it must be worn by the age of 24. While wearing the earring, any other male wearing it may challenge the bearer to Khanghaji and the two may engage wherever they choose. The act is considered to be sacred, and must not be interrupted, often drawing a crowd of spectators. Once completed, if the victor chose to transform the loser, each must remove their earrings and never wear them again.