During the decolonization of the British Empire, the UK did not want large numbers of non-white people immigrating to the British Isles. HK was only one instance of this. BN(O) nationality was one of many convenient bureaucratic fictions created in order to rationalize how someone born as a British subject in a British territory could lose the right to continue to live on British territory. Particularly affected were people of Indian ethnicity who had immigrated to other British colonies as civil servants; they became ethnic minorities in their newly independent countries and while their white coworkers were granted British citizenship, they were left behind.
Thousands of people became stateless as a result of this policy.
[1] https://en.wikipedia.org/wiki/British_Overseas_citizen [2] https://en.wikipedia.org/wiki/British_National_(Overseas)